[generator] fix generic parameter code generation for interface.

This fixes bug #43513.

The background for the fix is, well, complicated.

It was generating wrong code:

    // This method is explicitly implemented as a member of an instantiated Com.Google.Android.Exoplayer.Drm.IExoMediaDrm
    void global::Com.Google.Android.Exoplayer.Drm.IExoMediaDrm.SetOnEventListener (global::Com.Google.Android.Exoplayer.Drm.IExoMediaDrmOnEventListener p0)
    {
        SetOnEventListener (global::Java.Interop.JavaObjectExtensions.JavaCast<global::Com.Google.Android.Exoplayer.Drm.IExoMediaDrmOnEventListener<Java.Lang.Object>>(p0));
    }

where IExoMediaDrmOnEventListener is a normally generated interface ACW
which does not have any generic parameter.

The problem is, the Java API has generic argument T.

Usually it does not matter in normal classes. But reference to an interface
type with generic arguments is treated special: `IRequireGenericMarshal`

It was introduced at old monodroid commit 443598f (which is not in this
Java.Interop repo) for fixing bug #5979 to handle IList and ICollection
for collection marshaling.

So... basically ACWs were not expected to behave as IRequireGenericMarshal.
And now that behavior bites us at bug #43513.

Now, how to fix this new bug? This change introduces another boolean flag
to determine if reference to this type should really output generic
arguments. If it is False, then do not output any further.

This is the best logical explanation I can make right now.
What I still wonder are 1) whether it is only about InterfaceGen, and
2) if we could rather simplify things and eliminate this
IRequireGenericMarshal thing. But so far I have no answer to those.

Author: atsushieno

tools/generator/CollectionSymbol.cs

@@ -52,6 +52,10 @@ public string ElementType {
 			get { return null; }
 		}
 
+		public bool MayHaveManagedGenericArguments {
+			get { return true; }
+		}
+
 		public string GetObjectHandleProperty (string variable)
 		{
 			return $"((global::Java.Lang.Object) {variable}).Handle";

tools/generator/GenericSymbol.cs

@@ -91,7 +91,8 @@ public string FromNative (CodeGenerationOptions opt, string varname, bool owned)
 
 		public string GetGenericType (Dictionary<string, string> mappings)
 		{
-			return gen.FullName + (mappings == null ? tps : MapTypeParams (mappings));
+			var rgm = gen as IRequireGenericMarshal;
+			return gen.FullName + (rgm != null && !rgm.MayHaveManagedGenericArguments ? null : mappings == null ? tps : MapTypeParams (mappings));
 		}
 
 		public string ToNative (CodeGenerationOptions opt, string varname)

tools/generator/IRequireGenericMarshal.cs

@@ -5,6 +5,7 @@ namespace MonoDroid.Generation
 {
 	public interface IRequireGenericMarshal
 	{
+		bool MayHaveManagedGenericArguments { get; }
 		string GetGenericJavaObjectTypeOverride ();
 		string ToInteroperableJavaObject (string varname);
 	}

tools/generator/InterfaceGen.cs

@@ -24,9 +24,14 @@ public ManagedInterfaceGen (TypeDefinition t)
 				AddMethod (new ManagedMethod (this, m));
 			}
 		}
+
 		public override string ArgsType {
 			get { throw new NotImplementedException (); }
 		}
+
+		public override bool MayHaveManagedGenericArguments {
+			get { return !this.IsAcw; }
+		}
 	}
 #endif
 
@@ -109,6 +114,10 @@ public bool IsListener {
 			get { return Name.EndsWith ("Listener") && Properties.Count == 0 && Interfaces.Count == 0; }
 		}
 
+		public virtual bool MayHaveManagedGenericArguments {
+			get { return false; }
+		}
+
 		public override string NativeType {
 			get { return "IntPtr"; }
 		}