Properly generate systemUiVisibilityChanged callbacks.

User activity is supposed to immediately revert
HIDE_NAVIGATION, but we were just showing the navigation bar
directly. By routing through the service we'll generate
callbacks for everyone.

At long last, SYSTEM_UI_FLAG_HIDE_NAVIGATION will work like
lights out (now called SYSTEM_UI_FLAG_LOW_PROFILE) which
worked well enough in Honeycomb.

Bug: 5052456
Change-Id: I677f1295e1208c1aec48a4a9c8a6850916688388