SurfaceFlinger: VSyncPredictor should be robust with inconsistent vsyncs

If VSyncPredictor gets an invalid timestamp, it might be accepted as
a correct one, and then all the other correct timestamps will become
incorrect for VSyncPredictor leading to rejecting all the correct
timestamps followed by a one off inconsistent vsync.

Bug: 173235499
Test: SF unit tests (+1 new)
Change-Id: I192b917196c727b9a97a0d18bae160467f75722d