Optimize system bar background drawable.

Out with TransitionDrawable.

The new background drawable knows about all possible
background styles, and optimizes the transitions
between them - including picking up from the current
state, force finishing on screen off, and using
SystemClock.elapsedRealtime() for timing.

Bug:11254317
Change-Id: Ice83dc966f6674ef97f7008f2a1b62d67ec59e7d