Reduce screen on/off latency.

Reduce latency of screen on/off and improve how it is synchronized with
backlight changes.  Screen state changes are no longer posted to vsync
which should save time.  What's more, the state change occurs on a
separate thread so we no longer run the risk of blocking the Looper
for a long time while waiting for the screen to turn on or off.

Bug: 7382919
Bug: 7139924
Change-Id: I375950d1b07e22fcb94efb82892fd817e2f780dc