Revised "immersive mode" API.

No longer a window bit, FLAG_IMMERSIVE is now set on
ActivityInfo.flags and in the Activity's manifest as
android:immersive="true" (ActivityInfo).

[An "immersive" activity is one that wishes to avoid being
paused by full-screen notifications (like an incoming call).
An activity that sets FLAG_IMMERSIVE/android:immersive is
sending a signal to the notification manager, status bar,
etc. that they should try to find some other way to get the
user's attention in high-priority situations.]

[Originally: change Ie290c2e.]

Change-Id: I967bb10b930b8f0772b10f81f2957a03fa3f1736