fix multi Activity support

jobjects have to be compared with env->IsSameObject()
and each Activity needs its own Window instance.
The GtkWindow is still shared between all Activities

src/api-impl/android/view/Window.java

@@ -20,6 +20,7 @@ public class Window {
 
 	// FIXME private
 	public long native_window;
+	public View contentView;
 
 	private Window.Callback callback;
 
@@ -39,6 +40,7 @@ public class Window {
 	}
 
 	public void setContentView(View view) {
+		contentView = view;
 		set_widget_as_root(native_window, view.widget);
 	}