Introduce "--fullscreen" command line option for xwalk-launcher

Introduce "--fullscreen" command line option for xwalk-launcher
in order to allow launching applications in fullscreen mode
without necessity to re-start xwalk service process. Besides,
it allows simultaneous running applications in both fullscreen
and default modes.

This patch also moves the logic enabling fullscreen mode from the
Runtime class to more appropriate places: ApplicationSystem
and RunningApplicationsManager.

BUG=XWALK-1229