Fix issue #3400119: API to specify a black background behind a window transition

There is now an API, which is used for task switching.

Also improved how we handle rotation animation when we can't take a
screen shot, to cleanly revert to the old freeze behavior.  This removes
the need to special case the emulator.

Change-Id: I7227432a2309370437ec6ac78db02c6f1e7eedd5