SF: vsync divisors should use the same reference point

Fix an issue where vsync divisors use different reference points
to classify whether a vsync is in phase or not. This caused an
issue where 30fps and 60fps might picked a difference reference point,
causing a 30fps vsync to be out of phase with a 60fps vsync.

Test: SF unit tests
Bug: 267780202
Change-Id: I95939670f2850e90978a816d319a59f7c6fa7ba5