SurfaceFlinger: Do less work when using PTS

Currently, SurfaceFlinger is very dumb about how it handles buffer
updates at less than 60fps. If there is a new frame pending, but its
timestamp says not to present it until later SurfaceFlinger will wake
up every vsync until it is time to present it. Even worse, if
SurfaceFlinger has woken up but nothing has changed, it still goes
through the entire composition process.

This change (mostly) fixes that inefficiency. SurfaceFlinger will
still wake up every refresh period while there is a new frame
pending, but if there is no work to do, it will almost immediately go
back to sleep.

Bug: 18111837
Change-Id: I7825bacd37f40bf26edcc6a5e0f051dce45291fb