SF: do not clear mLastVsyncSequence on VSyncPredictor::clearTimestamps

Clearing it looses the information about the last reference point.
Even if that point is incorrect, it is still better than nothing,
and the logic will self correct itself once the model gets more timestamps.

Test: atest FrameRateOverrideHostTest
Bug: 269561042
Change-Id: Ic0a1a173b5f99820e93e31a0dba11ee2b71cb6f0