[One .NET] fix more Mono.Android.dll linker warnings (#6270)

Context: dotnet/linker#2265
Context: #5652

There are two remaining linker warnings when building with
`$(SuppressTrimAnalysisWarnings)`=false:

	> dotnet build -c Release -p:SuppressTrimAnalysisWarnings=false
	…
	src\Mono.Android\Android.Runtime\ResourceIdManager.cs(37,6): warning IL2026: Android.Runtime.ResourceIdManager.GetResourceTypeFromAssembly(Assembly): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.
	src\Mono.Android\Java.Interop\JavaObjectExtensions.cs(136,4): warning IL2026: Java.Interop.JavaObjectExtensions.GetHelperType(Type,String): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.
	src\Mono.Android\Java.Interop\JavaObjectExtensions.cs(131,5): warning IL2026: Java.Interop.JavaObjectExtensions.GetHelperType(Type,String): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.

The two issues here:

 1. `ResourceIdManager` uses reflection for the implementation of
    `Resource.designer.cs`.  Use `[UnconditionalSuppressMessage]` to
    ignore the warning, because the assembly hitting the warning is
    the "root assembly", which is *not linked*.  The methods in
    question will not be removed by the linker.

 2. `JavaObjectExtensions.GetHelperType()` dynamically looks up
    `*Invoker` types.  Use `[UnconditionalSuppressMessage]` to ignore
    the warning, because the `MarkJavaObjects()` linker step preserves
    them.  Additionally, I renamed the method to `GetInvokerType()`
    and hardcoded `"Invoker"` within the method.  This should prevent
    us from messing this up if a new "helper" type is added in
    addition to `Invoker`.

Unfortunately, even after all the issues are solved. `ILLink` still
appears to emit warnings:

	Java.Interop.dll warning IL2104: Assembly 'Java.Interop' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries
	Mono.Android.dll warning IL2104: Assembly 'Mono.Android' produced trim warnings. For more information see https://aka.ms/dotnet-illink/libraries

Adding `-p:TrimmerSingleWarn=false` shows an additional 30 warnings.

src/Mono.Android/Android.Runtime/ResourceIdManager.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 
 namespace Android.Runtime
@@ -30,6 +31,7 @@ public static void UpdateIdValues ()
 			}
 		}
 
+		[UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "Types in Resource.designer.cs are preserved, because it is the root assembly passed to the linker.")]
 		static Type? GetResourceTypeFromAssembly (Assembly assembly)
 		{
 			foreach (var customAttribute in assembly.GetCustomAttributes (typeof (ResourceDesignerAttribute), true)) {

src/Mono.Android/Java.Interop/JavaObjectExtensions.cs

@@ -92,7 +92,7 @@ static IJavaObject CastClass (IJavaObject instance, Type resultType)
 
 			if (resultType.IsAbstract) {
 				// TODO: keep in sync with TypeManager.CreateInstance() algorithm
-				var invokerType = GetHelperType (resultType, "Invoker");
+				var invokerType = GetInvokerType (resultType);
 				if (invokerType == null)
 					throw new ArgumentException ("Unable to get Invoker for abstract type '" + resultType.FullName + "'.", "TResult");
 				resultType = invokerType;
@@ -122,10 +122,12 @@ static IJavaObject CastClass (IJavaObject instance, Type resultType)
 							instance.GetType ().FullName, resultType.FullName));
 		}
 
-		// typeof(Foo) -> FooSuffix
-		// typeof(Foo<>) -> FooSuffix`1
-		internal static Type? GetHelperType (Type type, string suffix)
+		// typeof(Foo) -> FooInvoker
+		// typeof(Foo<>) -> FooInvoker`1
+		[UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "*Invoker types are preserved by the MarkJavaObjects linker step.")]
+		internal static Type? GetInvokerType (Type type)
 		{
+			const string suffix = "Invoker";
 			Type[] arguments = type.GetGenericArguments ();
 			if (arguments.Length == 0)
 				return type.Assembly.GetType (type + suffix);

src/Mono.Android/Java.Interop/TypeManager.cs

@@ -292,7 +292,7 @@ internal static IJavaPeerable CreateInstance (IntPtr handle, JniHandleOwnership
 				type = targetType;
 
 			if (type.IsInterface || type.IsAbstract) {
-				var invokerType = JavaObjectExtensions.GetHelperType (type, "Invoker");
+				var invokerType = JavaObjectExtensions.GetInvokerType (type);
 				if (invokerType == null)
 					throw new NotSupportedException ("Unable to find Invoker for type '" + type.FullName + "'. Was it linked away?",
 							CreateJavaLocationException ());

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/MarkJavaObjects.cs

@@ -59,7 +59,6 @@ void PreserveJavaObjectImplementation (TypeDefinition type)
 		{
 			PreserveIntPtrConstructor (type);
 			PreserveAttributeSetConstructor (type);
-			PreserveAdapter (type);
 			PreserveInvoker (type);
 		}
 
@@ -151,24 +150,6 @@ void PreserveMethod (TypeDefinition type, MethodDefinition method)
 			Annotations.AddPreservedMethod (type, method);
 		}
 
-		void PreserveAdapter (TypeDefinition type)
-		{
-			var adapter = PreserveHelperType (type, "Adapter");
-
-			if (adapter == null || !adapter.HasMethods)
-				return;
-
-			foreach (MethodDefinition method in adapter.Methods) {
-				if (method.Name != "GetObject")
-					continue;
-
-				if (method.Parameters.Count != 2)
-					continue;
-
-				PreserveMethod (type, method);
-			}
-		}
-
 		string TypeNameWithoutKey (string name)
 		{
 			var idx = name.IndexOf (", PublicKeyToken=");
@@ -208,25 +189,18 @@ void PreserveInterfaceMethods (TypeDefinition type, TypeDefinition invoker)
 
 		void PreserveInvoker (TypeDefinition type)
 		{
-			var invoker = PreserveHelperType (type, "Invoker");
+			var invoker = GetInvokerType (type);
 			if (invoker == null)
 				return;
 
+			PreserveConstructors (type, invoker);
 			PreserveIntPtrConstructor (invoker);
 			PreserveInterfaceMethods (type, invoker);
 		}
 
-		TypeDefinition PreserveHelperType (TypeDefinition type, string suffix)
-		{
-			var helper = GetHelperType (type, suffix);
-			if (helper != null)
-				PreserveConstructors (type, helper);
-
-			return helper;
-		}
-
-		TypeDefinition GetHelperType (TypeDefinition type, string suffix)
+		TypeDefinition GetInvokerType (TypeDefinition type)
 		{
+			const string suffix = "Invoker";
 			string fullname = type.FullName;
 
 			if (type.HasGenericParameters) {

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/XASdkTests.cs

@@ -205,8 +205,14 @@ public void DotNetNew ([Values ("android", "androidlib", "android-bindinglib")]
 			File.WriteAllBytes (Path.Combine (dotnet.ProjectDirectory, "foo.jar"), ResourceData.JavaSourceJarTestJar);
 			Assert.IsTrue (dotnet.New ("android-activity"), "`dotnet new android-activity` should succeed");
 			Assert.IsTrue (dotnet.New ("android-layout", Path.Combine (dotnet.ProjectDirectory, "Resources", "layout")), "`dotnet new android-layout` should succeed");
+
+			// Debug build
 			Assert.IsTrue (dotnet.Build (), "`dotnet build` should succeed");
 			dotnet.AssertHasNoWarnings ();
+
+			// Release build
+			Assert.IsTrue (dotnet.Build (parameters: new [] { "Configuration=Release" }), "`dotnet build` should succeed");
+			dotnet.AssertHasNoWarnings ();
 		}
 
 		[Test]