Fix the layout issue happens when app was switched back from background.

In the activity onResume() method, the view enters fullscreen mode
without checking the Fullscreen property. An if condition was added
to fix the issue.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2187
(cherry picked from commit 6cce1f1d91c98a1456a020ab0005d00ac99a8fbb)