Adjust present time calculation

Adjust the target present time when using
render_ahead to better fit in the gaps between
vsync pulses. This prevents small clock
drifts from causing jank.

Bug: 143540628
Test: manual, looked at systrace
Change-Id: I26ae6ebf16a3dea48dfd8e1c420d9e5e43e42bcb
1 file changed