[Android] Create the XWalkRenderViewExt object.

XWalkRenderViewExt is a class that handles several IPCs from the class
XWalkRenderViewHostExt. To make it work, an object of XWalkRenderViewExt
must be created first.
Merge pull request #952 from gaochun/XWalkRenderViewExt

[Android] Create the XWalkRenderViewExt object.