[Android] Implement Screen Orientation API through extension (Part 2)

Expose entry points functionality for extension on Android platform, since
screen orientation api is needed to binding to name space "window.screen",
we are using set entry point functionality to implement it like what tizen
is doing.

Add a new construct for XWalkExtension, which won't affect existing extension.
Merge pull request #1501 from yinli/screen_orientation_api

[Android] Implement Screen Orientation API through extension (Part 2)