New API for "immersive" activity windows.

An "immersive" activity (as indicated by the new
FLAG_IMMERSIVE) is one that wishes to avoid being paused by
full-screen notifications (like an incoming call).
An activity that sets FLAG_IMMERSIVE on its window 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.

FLAG_IMMERSIVE should be used exclusively in conjunction
with FLAG_FULL_SCREEN (that is, only activities that hide
the status bar should consider themselves immersive).

Change-Id: Ie290c2e92fc391bcf55edfdb1fbd626cd284e3e2