Tweak RT-animator scheduling

Bug: 18226391

The issue occurs as a result of a dispatchFrame itself
taking longer than 12ms, the alloted budget. The result
is that a vsync request (which occured at the end) would
miss the vsync that occured 1ms prior to the end of the frame.
As a result it would end up waiting for the following vsync,
essentially dropping to 30fps even though 60 could have been
sustained.

Fix this with a few tweaks.
First, adjust the UI thread's delay bias from (now + 4ms) to
(vsync + 4ms), this prevents RT animators from slowly drifting
if the vsync occurs mid-task.
Second, request a vsync preemptively prior to running callbacks.
This way if any callbacks needs the next vsync and it takes
"too long", we will catch that vsync.
Finally, fix an issue where the display event queue was always
drained & rejected at the end of a task loop. Instead, drain
and reject all stale vsyncs. This still prevents the issue of
both UI thread & RT thread trying to drive 2 frames in a single
pulse, but also allows RT to notice that it missed a vsync
pulse it needed and that it should speed-up a bit in response

Change-Id: I9d6be037737e9283297898cac2e3563453e797cd