[Android] Expose fullscreen related API to XWalkView

It's for embedder to customize the behavior of fullscreen
exiting in a flexible way.

BUG=https://crosswalk-project.org/jira/browse/XWALK-954

It's cherry-picked from a73a3a6b.
Merge pull request #1584 from wang16/crosswalk-4

[Backport][Crosswalk-4][Android] Expose fullscreen related API to XWalkView