[Android] Enable W3C fullscreen API support

Fullscreen API spec is at http://www.w3.org/TR/fullscreen/.
The API allows web app to switch between fullscreen/non-fullscreen
dynamically in javascript. And also the ability to fullscreen a
dom element.

For Android, Content already provide related interface in WebContentsDelegate.
Implementing the interface and bridge to XWalkWebChromeClient finally to
toggle Activity's fullscreen status.

TEST=Manual
BUG=https://crosswalk-project.org/jira/browse/XWALK-773