Avoid re-calculating vsync mid-frame

Fixes: 29072773

By using computeFrameTime AnimationContext would
potentially end up modifying the latest vsync if
a very-slow frame was received from the UI thread.

This could potentially desync animations that were
RT & UI thread 'synchronized', but more significantly
it would confuse the swap chain which tries to only
draw one frame per vsync causing unneccessary frame
drops.

Change-Id: Ibd2ec3157ce32fee1eec8d56837c45a35e622895