[Java.Interop] Do not dispose live references on return to JNI

Fixes #374

Also refactor the code generation to not create `_handle` variable
when returning value to JNI

Example generated marshal method:

    using Android.OS;
    using Android.Views;
    using Java.Interop;
    using System;

    public static IntPtr n_onCreateView_Landroid_view_LayoutInflater_Landroid_view_ViewGroup_Landroid_os_Bundle_ (IntPtr __jnienv, IntPtr __this, IntPtr inflater, IntPtr container, IntPtr savedInstanceState)
    {
    	JniTransition jniTransition = new JniTransition (__jnienv);
    	JniRuntime runtime = default(JniRuntime);
    	global::Android.Views.View view = default(global::Android.Views.View);
    	try {
    		runtime = JniEnvironment.Runtime;
    		JniRuntime.JniValueManager valueManager = runtime.ValueManager;
    		valueManager.WaitForGCBridgeProcessing ();
    		FragmentContainer value = valueManager.GetValue<FragmentContainer> (__this);
    		LayoutInflater value2 = valueManager.GetValue<LayoutInflater> (inflater);
    		ViewGroup value3 = valueManager.GetValue<ViewGroup> (container);
    		Bundle value4 = valueManager.GetValue<Bundle> (savedInstanceState);
    		view = value.OnCreateView (value2, value3, value4);
    		JniObjectReference value5 = (view != null) ? view.PeerReference : default(JniObjectReference);
    		return JniEnvironment.References.NewReturnToJniRef (value5);
    	} catch (Exception ex) when (runtime.ExceptionShouldTransitionToJni (ex)) {
    		jniTransition.SetPendingException (ex);
    		return default(IntPtr);
    	} finally {
    		if (view != null) {
    			((IJavaPeerable)view).DisposeUnlessReferenced ();
    		}
    		jniTransition.Dispose ();
    	}
    	IntPtr intPtr = default(IntPtr);
    	return intPtr;
    }

Author: radekdoulik

src/Java.Interop/Java.Interop/JniRuntime.JniValueManager.cs

@@ -590,6 +590,17 @@ public override void DestroyGenericArgumentState (IJavaPeerable value, ref JniVa
 		}
 
 		public override Expression CreateParameterFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize)
+		{
+			var r = CreateIntermediaryExpressionFromManagedExpression (context, sourceValue);
+
+			var h = Expression.Variable (typeof (IntPtr), sourceValue + "_handle");
+			context.LocalVariables.Add (h);
+			context.CreationStatements.Add (Expression.Assign (h, Expression.Property (r, "Handle")));
+
+			return h;
+		}
+
+		Expression CreateIntermediaryExpressionFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue)
 		{
 			var r   = Expression.Variable (typeof (JniObjectReference), sourceValue.Name + "_ref");
 			context.LocalVariables.Add (r);
@@ -598,19 +609,19 @@ public override Expression CreateParameterFromManagedExpression (JniValueMarshal
 						test:       Expression.Equal (Expression.Constant (null), sourceValue),
 						ifTrue:     Expression.Assign (r, Expression.New (typeof (JniObjectReference))),
 						ifFalse:    Expression.Assign (r, Expression.Property (sourceValue, "PeerReference"))));
-			context.CleanupStatements.Add (DisposeObjectReference (r));
 
-			var h   = Expression.Variable (typeof (IntPtr), sourceValue + "_handle");
-			context.LocalVariables.Add (h);
-			context.CreationStatements.Add (Expression.Assign (h, Expression.Property (r, "Handle")));
-			return h;
+			if (typeof (IJavaPeerable).GetTypeInfo ().IsAssignableFrom (sourceValue.Type.GetTypeInfo ())) {
+				context.CleanupStatements.Add (Expression.IfThen (
+							test:       Expression.NotEqual (Expression.Constant (null), sourceValue),
+							ifTrue:     Expression.Call (sourceValue, typeof (IJavaPeerable).GetTypeInfo ().GetDeclaredMethod ("DisposeUnlessReferenced"))));
+			}
+
+			return r;
 		}
 
 		public override Expression CreateReturnValueFromManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue)
 		{
-			CreateParameterFromManagedExpression (context, sourceValue, 0);
-			var r   = context.LocalVariables [sourceValue + "_ref"];
-			return ReturnObjectReferenceToJni (context, sourceValue.Name, r);
+			return ReturnObjectReferenceToJni (context, sourceValue.Name, CreateIntermediaryExpressionFromManagedExpression (context, sourceValue));
 		}
 
 		public override Expression CreateParameterToManagedExpression (JniValueMarshalerContext context, ParameterExpression sourceValue, ParameterAttributes synchronize, Type targetType)