Don't clear timestamps when enabling HW vsync.

It should clear after the first HW Vsync timestamp comes in,
and when VSyncReactor period confirmed. Therefore, a prediction
when HW vsync is enabled but not yet confirmed is consistent with a
prediction when coming out of idle.

Bug: 272802278
Test: Flash, perfetto trace
Test: atest libsurfaceflinger_unittest
Change-Id: I619162d5583f33087de04c0e3b073a589ec4c2f7