Immersive activity API.

An Activity can declare itself to be "immersive" either by
setting android:immersive="true" in AndroidManifest or by
calling setImmersive(true).

Immersive activities "should" not be interrupted, for
example by Notifications with an associated
fullScreenIntent. (In the future we may even prevent any
non-system application from successfully calling
startActivity() if the foreground activity is immersive.)
Notifications with FLAG_HIGH_PRIORITY set will be shown to
the user in some less-obtrusive way if the frontmost
activity is immersive.

Change-Id: I8d0c25cc4e22371c27cbf2bb6372d2c95d57b2d7