SF: increase precision in VSyncPredictor

An issue was observed where the sf-app timeline woke up 1ms later than
it should have. Issue was tracked down to an imprecision in some of the
math in VSyncPredictor, which this issue corrects.

Fixes: 151146131
Test: 2 new unit tests, one of which was from bug
Test: flash dogfood device with patch

Change-Id: I1994b5b532e292a140d4736c7090b68224f75a02