[Java.Interop] Avoid some method group conversions (#1050)

Context: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-11#improved-method-group-conversion-to-delegate
Context: #1034
Context: dotnet/roslyn#62832

C#11 introduced a "slight semantic" change with "Improved method
group conversion to delegate":

> The C# 11 compiler caches the delegate object created from a method
> group conversion and reuses that single delegate object.

The result of optimization is that for current `generator`-emitted
code such as:

	static Delegate GetFooHandler ()
	{
	  if (cb_foo == null)
	    cb_foo = JNINativeWrapper.CreateDelegate ((_JniMarshal_PP_V) n_Foo);
	  return cb_foo;
	}

The `(_JniMarshal_PP_V) n_Foo` expression is a "method group
conversion", and under C#11 the generated IL is *larger*, as the
delegate instance is *cached* in case it is needed again.

However in *our* case we *know* the delegate instance won't be needed
again, not in this scope, so all this "optimization" does *for us* is
increase the size of our binding assemblies when built under  C#11.

Review `src/Java.Interop` for use of `new JniNativeMethodRegistration`
and replace "cast-style" method group conversions `(D) M` to
"new-style" delegate conversions `new D(M)`.  This explicitly
"opts-out" of the C#11 optimization.

src/Java.Interop/Java.Interop/JavaProxyObject.cs

@@ -17,9 +17,9 @@ sealed class JavaProxyObject : JavaObject, IEquatable<JavaProxyObject>
 		[JniAddNativeMethodRegistrationAttribute]
 		static void RegisterNativeMembers (JniNativeMethodRegistrationArguments args)
 		{
-			args.Registrations.Add (new JniNativeMethodRegistration ("equals",   "(Ljava/lang/Object;)Z", (EqualsMarshalMethod)Equals));
-			args.Registrations.Add (new JniNativeMethodRegistration ("hashCode", "()I",                   (GetHashCodeMarshalMethod)GetHashCode));
-			args.Registrations.Add (new JniNativeMethodRegistration ("toString", "()Ljava/lang/String;",  (ToStringMarshalMethod)ToString));
+			args.Registrations.Add (new JniNativeMethodRegistration ("equals",   "(Ljava/lang/Object;)Z", new EqualsMarshalMethod (Equals)));
+			args.Registrations.Add (new JniNativeMethodRegistration ("hashCode", "()I",                   new GetHashCodeMarshalMethod (GetHashCode)));
+			args.Registrations.Add (new JniNativeMethodRegistration ("toString", "()Ljava/lang/String;",  new ToStringMarshalMethod (ToString)));
 		}
 
 		public override JniPeerMembers JniPeerMembers {

src/Java.Interop/Java.Interop/ManagedPeer.cs

@@ -25,11 +25,11 @@ static ManagedPeer ()
 					new JniNativeMethodRegistration (
 						"construct",
 						ConstructSignature,
-						(ConstructMarshalMethod) Construct),
+						new ConstructMarshalMethod (Construct)),
 					new JniNativeMethodRegistration (
 						"registerNativeMembers",
 						RegisterNativeMembersSignature,
-						(RegisterMarshalMethod) RegisterNativeMembers)
+						new RegisterMarshalMethod (RegisterNativeMembers))
 			);
 		}