SurfaceFlinger: SW-based vsync events

This change adds the DispSync class, which models the hardware vsync event
times to allow vsync event callbacks to be done at an arbitrary phase offset
from the hardware vsync.  This can be used to reduce the minimum latency from
Choreographer wake-up to on-screen image presentation.

Bug: 10624956
Change-Id: I8c7a54ceacaa4d709726ed97b0dcae4093a7bdcf