[One .NET] remove in-tree DynamicDependencyAttribute

Context: dotnet#5652

In earlier development of .NET 6, we had a copy of
`DynamicDependencyAttribute` because we were compiling
`Mono.Android.dll` for `netcoreapp3.1`. Now we are compiling for
`net6.0`, so we can remove these files. We also can condition their
usage behind `#if NET`, to mean ".NET 5 or higher", where the
attributes were introduced.

I also fixed the linker warning:

    src\Mono.Android\Android.Runtime\AndroidRuntime.cs(296,5): warning IL2026: Android.Runtime.AndroidTypeManager.CreateDynamicCallback(MethodInfo): Using method 'System.Reflection.Assembly.GetType(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Types might be removed.

By introducing:

    [DynamicDependency ("Create", "Java.Interop.DynamicCallbackCodeGenerator", "Mono.Android.Export")]

However, we still need to suppress the warning:

    [UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "DynamicDependency should preserve the Create() method.")]

src/Mono.Android.Export/CallbackCode.cs

@@ -406,6 +406,7 @@ public CodeExpression FromNative (CodeExpression arg)
 			throw new InvalidOperationException ();
 		}
 
+#if NET
 		// Ignore ToNative() overload that takes generic instancing mapping. The reflected method should have nothing to do with it.
 		[DynamicDependency ("ToLocalJniHandle", "Android.Runtime.JavaArray`1", "Mono.Android")]
 		[DynamicDependency ("ToLocalJniHandle", "Android.Runtime.JavaCollection", "Mono.Android")]
@@ -416,6 +417,7 @@ public CodeExpression FromNative (CodeExpression arg)
 		[DynamicDependency ("ToLocalJniHandle", "Android.Runtime.JavaList`1", "Mono.Android")]
 		[DynamicDependency ("ToLocalJniHandle", "Android.Runtime.JavaSet", "Mono.Android")]
 		[DynamicDependency ("ToLocalJniHandle", "Android.Runtime.JavaSet`1", "Mono.Android")]
+#endif
 		public CodeExpression ToNative (CodeExpression arg)
 		{
 			switch (GetKind (type)) {

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading;
@@ -285,6 +286,10 @@ protected override IEnumerable<string> GetSimpleReferences (Type type)
 
 		static MethodInfo? dynamic_callback_gen;
 
+#if NET
+		[DynamicDependency ("Create", "Java.Interop.DynamicCallbackCodeGenerator", "Mono.Android.Export")]
+		[UnconditionalSuppressMessage ("Trimming", "IL2026", Justification = "DynamicDependency should preserve the Create() method.")]
+#endif
 		static Delegate CreateDynamicCallback (MethodInfo method)
 		{
 			if (dynamic_callback_gen == null) {

src/Xamarin.Android.NamingCustomAttributes/Java.Interop/ExportAttribute.cs

@@ -12,12 +12,16 @@ namespace Java.Interop {
 #endif  // !JCW_ONLY_TYPE_NAMES
 	partial class ExportAttribute : Attribute {
 
+#if NET
 		[DynamicDependency (DynamicallyAccessedMemberTypes.All, "Java.Interop.DynamicCallbackCodeGenerator", "Mono.Android.Export")]
+#endif
 		public ExportAttribute ()
 		{
 		}
-
+
+#if NET
 		[DynamicDependency (DynamicallyAccessedMemberTypes.All, "Java.Interop.DynamicCallbackCodeGenerator", "Mono.Android.Export")]
+#endif
 		public ExportAttribute (string name)
 		{
 			Name = name;

src/Xamarin.Android.NamingCustomAttributes/Java.Interop/ExportFieldAttribute.cs

@@ -12,7 +12,9 @@ namespace Java.Interop {
 #endif  // !JCW_ONLY_TYPE_NAMES
 	partial class ExportFieldAttribute : Attribute {
 
+#if NET
 		[DynamicDependency (DynamicallyAccessedMemberTypes.All, "Java.Interop.DynamicCallbackCodeGenerator", "Mono.Android.Export")]
+#endif
 		public ExportFieldAttribute (string name)
 		{
 			Name = name;

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

@@ -23,8 +23,4 @@
     <Compile Include="$(MSBuildThisFileDirectory)Android.App\LayoutAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Java.Interop\IJniNameProviderAttribute.cs" />
   </ItemGroup>
-  <ItemGroup Condition=" '$(TargetFramework)' != 'net6.0' ">
-    <Compile Include="$(MSBuildThisFileDirectory)System.Diagnostics.CodeAnalysis/DynamicallyAccessedMemberTypes.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System.Diagnostics.CodeAnalysis/DynamicDependencyAttribute.cs" />
-  </ItemGroup>
 </Project>