Make immersive mode public & imply update locking

Activity.setImmersive(boolean) / android:immersive="bool" are now public.
In addition, if the foreground activity is immersive then an update lock
will be held on its behalf.  This lets applications such as movie players
suppress the display of intrusive notifications, OTA-availability dialogs,
and the like while they are displaying content that ought not to be
rudely interrupted.

The update lock aspect of this mode is *advisory*, not binding -- the
update mechanism is not actually constrained; it simply uses this information
in deciding whether/when to prompt the user.  It's more a guideline than
a rule.

Bug 6154438

Change-Id: Ibd3491fc437077f3fa0d9708ed91955121e8c877