visibility ("lights out") API.

1. Views may setSystemUiVisibility() to recommend that
the system chrome (status bar or other UI) show or hide
itself.  (This functionality was previously available only
via the FLAG_FULLSCREEN window flag for some SystemUI
implementations.)

2. Views may register a OnSystemUiVisibilityChangedListener
on a view, and find out when the system UI actually
appears or disappears, allowing apps to coordinate the
appearance of their own UI if desired.

Bug: 3241144
Change-Id: Ia1758d94099182d49a1e3688ea2738ae4995b829