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
Merge pull request #131 from gaochun/2187

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