SF: update mTimestamps with the newest hwvsync timestamp

When a HWvsync timestamp is dropped since it doesn't match the cadence
of the previous ones, it may still be valid (especially when the older
ones are outdated). In that case update mKnownTimestamp with the new
timestamp.

Bug: 190422841
Test: Tap infrequently in TouchLatency and observe systraces
Change-Id: I10ec8dc1781c891ca280deb2e74bf6e17ce797c5