[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
[Android] Support back key to exit fullscreen

Previously, there is no way to exit fullscreen if developer
doesn't provide one. And back key is used to navigate backward.

Move the handling of back key into xwalk core and try to exit
fullscreen prior to navigate backward.
Merge pull request #1446 from wang16/fullscreen_html5

[Android] Enable W3C fullscreen API support