[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
Is there any possibility that the instance was not created internally and the class itself was create internally? Is there wiki to explain the reason to identify  internally and externally? Thanks.