Synthesize fake vsyncs when the screen is off.

When the screen is off, we might not receive real vsync pulses from
the hardware which would cause posted Choreographer callbacks to not run.
This is bad because messages in the Looper might be blocked behind a barrier
that is scheduled to be removed by one of those Choreographer callback
(see ViewRootImpl.doTraversals).  Until the barrier is removed, those messages
will not run.  To prevent starvation of the Looper, we synthesize fake vsync
pulses at a reduced rate whenever the display hardware stops generating them.

This change should fix a variety of rare non-deterministic bugs where
the system might appear to be unresponsive while the screen is off,
and spurious ANRs reported shortly after the screen is turned back on.

Bug: 6574842
Bug: 6636995
Bug: 6643559
Change-Id: I263f2fdf979afd79e5ac47a0cc5d34a93b860c21