Pie controls: Fix flickering nav bar in full screen

On rare occasions the pie service will deadlock. When this
happens, full screen mode can no longer hide the
navigation bar, since it is masked out internally.

This commit switchs from a self cooked mask mechanism within
the WindowManager to the private flag
WindowManager.LayoutParams.PRIVATE_FLAG_FORCE_SHOW_NAV_BAR.

Note that this will avoid flickering when pie service get
stuck in the activation path, but it will not resolve the
deadlock.

Change-Id: I2b64867081b37d9d6070df8086975e4327d97f5d