[Android] Add the method of onWindowFocusChanged() in app template.

The Crosswalk test script is using the app template activity java file
via packaging tool, in Android4.4 immersive sticky full screen mode,
this method of onWindowFocusChanged() is required for the testing
activity, this fix adds this method to enable the swiping up from
the bottom for fullscreen immersive sticky mode.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1200