[One .NET] fix remaining Mono.Android.dll linker warnings

Fixes: dotnet#5652

There are two remaining linker warnings when building with:

    > 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`. Specify `UnconditionalSuppressMessage` to
   ignore the warning, because the assembly hitting the warning is the
   "root assembly". The methods in question will not be removed by the
   linker.

2. `JavaObjectExtensions.GetHelperType` dynamically looks up `Invoker`
   types. Specify `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

My guess is that `UnconditionalSuppressMessage` isn't clearing
`IL2104` when it should be. I'll file an issue upstream in mono/linker
about this.

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

@@ -200,12 +200,25 @@ public Foo ()
 		[Test]
 		public void DotNetNew ([Values ("android", "androidlib", "android-bindinglib")] string template)
 		{
+			var parameters = new List<string> {
+				// TODO: Uncomment after warnings are solved
+				// 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
+				//"SuppressTrimAnalysisWarnings=false"
+			};
 			var dotnet = CreateDotNetBuilder ();
 			Assert.IsTrue (dotnet.New (template), $"`dotnet new {template}` should succeed");
 			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");
-			Assert.IsTrue (dotnet.Build (), "`dotnet build` should succeed");
+
+			// Debug build
+			Assert.IsTrue (dotnet.Build (parameters: parameters.ToArray ()), "`dotnet build` should succeed");
+			dotnet.AssertHasNoWarnings ();
+
+			// Release build
+			parameters.Add ("Configuration=Release");
+			Assert.IsTrue (dotnet.Build (parameters: parameters.ToArray ()), "`dotnet build` should succeed");
 			dotnet.AssertHasNoWarnings ();
 		}