[Android] Clear FLAG_FORCE_NOT_FULLSCREEN when entering fullscreen

The FLAG_FORCE_NOT_FULLSCREEN flag of Android Window will force the
Android system to show status bar, even with SYSTEM_UI_FLAG_FULLSCREEN
set in SystemUIVisibility.

Record whether the FLAG_FORCE_NOT_FULLSCREEN is set before entering
fullscreen and clear it if so. Add the flag back if necessary when
exiting fullscreen.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2150