[Android] Allow XWalkView to use TextureView as backend for animation

* Add a preference key ANIMATABLE_XWALK_VIEW to enable animation on XWalkView
* Update XWalkView API documentation

BUG=https://crosswalk-project.org/jira/browse/XWALK-1574
[Android] Add sample code for animatable XWalkView usage

Alpha and scale X/Y is animated on the XWalkView in the sample code.
Merge pull request #1911 from hmin/texture-view-pref

[Android] Allow XWalkView to use TextureView as backend for animation