[XAT.Bytecode] Bind Kotlin internal interfaces as package-private (#645)

Context: dotnet/android#4661

In 439bd83, we made a change to hide Kotlin `internal` members as we
shouldn't be binding them.  However a `public class` can inherit from
an `internal interface`, so we need to emit those interfaces.

Instead, emit `internal interface` types as "package private" types;
`visibility=""` in `api.xml` parlance.  This allows us to resolve the
`interface` and thus successfully bind the `class`.  

In C#-land, the `interface` is not actually bound and the C# class
doesn't actually implement it.  This has no effect to the public API
end user, who shouldn't be seeing the `internal interface` anyways.
If a user really wants to publicly expose the `interface`, its
`visibility` can be changed via `metadata`.

***Note***: Changing the `visibility` of a type may result in
subsequent Java Callable Wrapper generation errors, if/when a
Java Callable Wrapper attempts to implement the package-private type.

src/Xamarin.Android.Tools.Bytecode/Kotlin/KotlinFixups.cs

@@ -66,6 +66,16 @@ static void FixupClassVisibility (ClassFile klass, KotlinClass metadata)
 		{
 			// Hide class if it isn't Public/Protected
 			if (klass.AccessFlags.IsPubliclyVisible () && !metadata.Visibility.IsPubliclyVisible ()) {
+
+				// Interfaces should be set to "package-private", which is "no visibility flags"
+				if (klass.AccessFlags.HasFlag (ClassAccessFlags.Interface)) {
+					Log.Debug ($"Kotlin: Setting interface {klass.ThisClass.Name.Value} to package-private");
+					klass.AccessFlags = (klass.AccessFlags ^ ClassAccessFlags.Public) & klass.AccessFlags;
+					klass.AccessFlags = (klass.AccessFlags ^ ClassAccessFlags.Protected) & klass.AccessFlags;
+					klass.AccessFlags = (klass.AccessFlags ^ ClassAccessFlags.Private) & klass.AccessFlags;
+					return;
+				}
+
 				Log.Debug ($"Kotlin: Hiding internal class {klass.ThisClass.Name.Value}");
 				klass.AccessFlags = ClassAccessFlags.Private;
 				return;

tests/Xamarin.Android.Tools.Bytecode-Tests/KotlinFixupsTests.cs

@@ -23,6 +23,21 @@ public void HideInternalClass ()
 			Assert.False (klass.AccessFlags.HasFlag (ClassAccessFlags.Public));
 		}
 
+		[Test]
+		public void MakeInternalInterfacePackagePrivate ()
+		{
+			var klass = LoadClassFile ("InternalInterface.class");
+
+			Assert.True (klass.AccessFlags.HasFlag (ClassAccessFlags.Public));
+
+			KotlinFixups.Fixup (new [] { klass });
+
+			// "package-private" is denoted as no visibility flags
+			Assert.False (klass.AccessFlags.HasFlag (ClassAccessFlags.Public));
+			Assert.False (klass.AccessFlags.HasFlag (ClassAccessFlags.Protected));
+			Assert.False (klass.AccessFlags.HasFlag (ClassAccessFlags.Private));
+		}
+
 		[Test]
 		public void HideInternalConstructor ()
 		{

tests/Xamarin.Android.Tools.Bytecode-Tests/kotlin/InternalInterface.kt

@@ -0,0 +1 @@
+internal interface InternalInterface