[Android] Fix the issue about onJavascriptModalDialog was not called.

This patch is to fix the issue about onJavascriptModalDialog was not called
when this function was overridden in class of XWalkUIClient.
Modify the condition for override function, if parameter is not the
instance of specified, create a new instance.

BUG=XWALK-2534
if there is no instance and the class itself is create internally, here it will return false with your code.