[Xamarin.Android.Build.Tasks] fix NRT warnings & other warnings (#9483)

Introduces the setting to `Xamarin.Android.Build.Tasks.csproj`:

    <WarningsAsErrors>Nullable</WarningsAsErrors>

Then I fixed the existing warnings. Down the road this will make it
easier to adopt `#enable nullable`, and eventually `Nullable=enable`
the entire project. Trying to do that now results in 1,000+ warnings...

Most of these are types that could easily be null, and they weren't
marked as nullable with `?`. Luckily, the code appears to be checking
for null in these places already.

For `*Attribute` types that need `IJniNameProviderAttribute.Name`,
I explicitly implemented a non-nullable version of this property in
`*Attribute.Partial.cs` files.

src/Xamarin.Android.Build.Tasks/Mono.Android/ActivityAttribute.Partial.cs

@@ -16,9 +16,9 @@ namespace Android.App {
 
 	partial class ActivityAttribute {
 
-		string _ParentActivity;
-		TypeDefinition type;
-		ICollection<string> specified;
+		string? _ParentActivity;
+		TypeDefinition? type;
+		ICollection<string>? specified;
 
 		static partial void AddManualMapping ()
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/ApplicationAttribute.Partial.cs

@@ -18,10 +18,10 @@ namespace Android.App {
 
 	partial class ApplicationAttribute {
 
-		string _BackupAgent;
-		ICustomAttributeProvider provider;
+		string? _BackupAgent;
+		ICustomAttributeProvider? provider;
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		static partial void AddManualMapping ()
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/BroadcastReceiverAttribute.Partial.cs

@@ -12,7 +12,7 @@ namespace Android.Content {
 
 	partial class BroadcastReceiverAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static BroadcastReceiverAttribute FromTypeDefinition (TypeDefinition type, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/ContentProviderAttribute.Partial.cs

@@ -28,7 +28,7 @@ static string[] ToStringArray (object value)
 			return values.Select (v => (string) v.Value).ToArray ();
 		}
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static ContentProviderAttribute FromTypeDefinition (TypeDefinition type, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/GrantUriPermissionAttribute.Partial.cs

@@ -12,7 +12,7 @@ namespace Android.Content {
 
 	partial class GrantUriPermissionAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<GrantUriPermissionAttribute> FromTypeDefinition (TypeDefinition type, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/InstrumentationAttribute.Partial.cs

@@ -12,7 +12,7 @@ namespace Android.App {
 
 	partial class InstrumentationAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<InstrumentationAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/LayoutAttribute.Partial.cs

@@ -17,8 +17,8 @@ namespace Android.App
 
 	partial class LayoutAttribute
 	{
-		TypeDefinition type;
-		ICollection<string> specified;
+		TypeDefinition? type;
+		ICollection<string>? specified;
 
 		public static LayoutAttribute FromTypeDefinition (TypeDefinition type, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/MetaDataAttribute.Partial.cs

@@ -12,7 +12,7 @@ namespace Android.App {
 
 	partial class MetaDataAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<MetaDataAttribute> FromCustomAttributeProvider (ICustomAttributeProvider type, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/PermissionAttribute.Partial.cs

@@ -17,7 +17,7 @@ namespace Android.App {
 
 	partial class PermissionAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<PermissionAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/PermissionGroupAttribute.Partial.cs

@@ -17,7 +17,7 @@ namespace Android.App {
 
 	partial class PermissionGroupAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<PermissionGroupAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/PermissionTreeAttribute.Partial.cs

@@ -17,7 +17,7 @@ namespace Android.App {
 
 	partial class PermissionTreeAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<PermissionTreeAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/PropertyAttribute.Partial.cs

@@ -12,7 +12,7 @@ namespace Android.App {
 
 	partial class PropertyAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<PropertyAttribute> FromCustomAttributeProvider (ICustomAttributeProvider type, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/ServiceAttribute.Partial.cs

@@ -14,7 +14,7 @@ namespace Android.App {
 
 	partial class ServiceAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static ServiceAttribute FromTypeDefinition (TypeDefinition type, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/SupportsGLTextureAttribute.Partial.cs

@@ -25,7 +25,7 @@ internal XElement ToElement (string packageName, TypeDefinitionCache cache)
 			return mapping.ToElement (this, specified, packageName, cache);
 		}
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<SupportsGLTextureAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/UsesConfigurationAttribute.Partial.cs

@@ -16,7 +16,7 @@ internal XElement ToElement (string packageName, TypeDefinitionCache cache)
 			return mapping.ToElement (this, specified, packageName, cache);
 		}
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<UsesConfigurationAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/UsesFeatureAttribute.Partial.cs

@@ -31,7 +31,7 @@ internal XElement ToElement (string packageName, TypeDefinitionCache cache)
 			return mapping.ToElement (this, specified, packageName, cache);
 		}
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<UsesFeatureAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/UsesLibraryAttribute.Partial.cs

@@ -12,7 +12,7 @@ namespace Android.App {
 
 	partial class UsesLibraryAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<UsesLibraryAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Mono.Android/UsesPermissionAttribute.Partial.cs

@@ -12,7 +12,7 @@ namespace Android.App {
 
 	partial class UsesPermissionAttribute {
 
-		ICollection<string> specified;
+		ICollection<string>? specified;
 
 		public static IEnumerable<UsesPermissionAttribute> FromCustomAttributeProvider (ICustomAttributeProvider provider, TypeDefinitionCache cache)
 		{

src/Xamarin.Android.Build.Tasks/Tasks/GetAotArguments.cs

@@ -149,7 +149,7 @@ int GetNdkApiLevel (NdkTools ndk, AndroidTargetArch arch)
 				// level already set
 			} else {
 				// Probably not ideal!
-				level       = MonoAndroidHelper.SupportedVersions.MaxStableVersion.ApiLevel;
+				level       = MonoAndroidHelper.SupportedVersions.MaxStableVersion?.ApiLevel ?? 21;
 			}
 
 			// Some Android API levels do not exist on the NDK level. Workaround this my mapping them to the
@@ -353,7 +353,7 @@ string GetLdFlags (NdkTools ndk, AndroidTargetArch arch, int level, string toolP
 			return ldFlags.ToString ();
 		}
 
-		static string GetNdkToolchainLibraryDir (NdkTools ndk, string binDir, string archDir = null)
+		static string GetNdkToolchainLibraryDir (NdkTools ndk, string binDir, string? archDir = null)
 		{
 			var baseDir = Path.GetFullPath (Path.Combine (binDir, ".."));
 

src/Xamarin.Android.Build.Tasks/Utilities/ManifestDocumentElement.cs

@@ -77,16 +77,16 @@ public ManifestDocumentElement (string element)
 
 		class MappingInfo {
 			public string             AttributeName;
-			public Func<T, object>    Getter;
-			public Action<T, object>  Setter;
+			public Func<T, object?>    Getter;
+			public Action<T, object>?  Setter;
 			public Type               MemberType;
 			public Func<T, string>    AttributeValue;
 			public Func<T, ICustomAttributeProvider, IAssemblyResolver, TypeDefinitionCache, string>    AttributeValue2;
 		}
 
 		readonly IDictionary<string, MappingInfo>   Mappings = new Dictionary<string, MappingInfo> ();
 
-		public void Add (string member, string attributeName, Func<T, object> getter, Action<T, object> setter, Type memberType = null)
+		public void Add (string member, string attributeName, Func<T, object?> getter, Action<T, object>? setter, Type memberType = null)
 		{
 			Mappings.Add (member, new MappingInfo {
 					AttributeName   = attributeName,

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj

@@ -22,6 +22,7 @@
     <Nullable>annotations</Nullable>
     <!-- Causes issues with linker files imported from Mono -->
     <NoWarn>$(NoWarn);CA1310;CA1305</NoWarn>
+    <WarningsAsErrors>Nullable</WarningsAsErrors>
   </PropertyGroup>
 
   <ItemGroup>

src/Xamarin.Android.NamingCustomAttributes/Android.App/ActivityAttribute.Partial.cs

@@ -1,10 +1,13 @@
 using System;
 using Android.Views;
+using Java.Interop;
 
 namespace Android.App;
 
 public sealed partial class ActivityAttribute
 {
+	string IJniNameProviderAttribute.Name => Name ?? "";
+
 	[Obsolete ("There is no //activity/@android:layoutDirection attribute. This was a mistake. " +
 	    "Perhaps you wanted ConfigurationChanges=ConfigChanges.LayoutDirection?")]
 	public LayoutDirection LayoutDirection { get; set; }

src/Xamarin.Android.NamingCustomAttributes/Android.App/ActivityAttribute.cs

@@ -397,8 +397,10 @@ static ActivityAttribute ()
 		mapping.Add (
 			member: "ShowOnLockScreen",
 			attributeName: "showOnLockScreen",
+#pragma warning disable CS0618 // Type or member is obsolete
 			getter: self => self.ShowOnLockScreen,
 			setter: (self, value) => self.ShowOnLockScreen = (bool) value
+#pragma warning restore CS0618 // Type or member is obsolete
 		);
 		mapping.Add (
 			member: "ShowWhenLocked",

src/Xamarin.Android.NamingCustomAttributes/Android.App/ApplicationAttribute.Partial.cs

@@ -0,0 +1,8 @@
+using Java.Interop;
+
+namespace Android.App;
+
+public sealed partial class ApplicationAttribute
+{
+	string IJniNameProviderAttribute.Name => Name ?? "";
+}

src/Xamarin.Android.NamingCustomAttributes/Android.App/InstrumentationAttribute.Partial.cs

@@ -0,0 +1,8 @@
+using Java.Interop;
+
+namespace Android.App;
+
+public sealed partial class InstrumentationAttribute
+{
+	string IJniNameProviderAttribute.Name => Name ?? "";
+}

src/Xamarin.Android.NamingCustomAttributes/Android.App/ServiceAttribute.Partial.cs

@@ -0,0 +1,8 @@
+using Java.Interop;
+
+namespace Android.App;
+
+public sealed partial class ServiceAttribute
+{
+	string IJniNameProviderAttribute.Name => Name ?? "";
+}

src/Xamarin.Android.NamingCustomAttributes/Android.Content/BroadcastReceiverAttribute.Partial.cs

@@ -0,0 +1,8 @@
+using Java.Interop;
+
+namespace Android.Content;
+
+public partial class BroadcastReceiverAttribute
+{
+	string IJniNameProviderAttribute.Name => Name ?? "";
+}

src/Xamarin.Android.NamingCustomAttributes/Android.Content/ContentProviderAttribute.Partial.cs

@@ -1,4 +1,5 @@
 using System;
+using Java.Interop;
 
 namespace Android.Content;
 
@@ -12,4 +13,6 @@ public ContentProviderAttribute (string [] authorities)
 			throw new ArgumentException ("At least one authority must be specified.", "authorities");
 		Authorities = authorities;
 	}
+
+	string IJniNameProviderAttribute.Name => Name ?? "";
 }

src/Xamarin.Android.NamingCustomAttributes/Xamarin.Android.NamingCustomAttributes.projitems

@@ -10,6 +10,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)Android.Content\BroadcastReceiverAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Android.Content\BroadcastReceiverAttribute.Partial.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.Content\ContentProviderAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.Content\ContentProviderAttribute.Partial.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.Runtime\RegisterAttribute.cs" />
@@ -20,8 +21,11 @@
     <Compile Include="$(MSBuildThisFileDirectory)Android.App\ActivityAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.App\ActivityAttribute.Partial.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.App\ApplicationAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Android.App\ApplicationAttribute.Partial.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.App\ServiceAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Android.App\ServiceAttribute.Partial.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.App\InstrumentationAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Android.App\InstrumentationAttribute.Partial.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Android.App\LayoutAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Java.Interop\IJniNameProviderAttribute.cs" />
   </ItemGroup>