SF: VSyncReactor change offsets at correct time

The VRR/MRR timing loop (currently flagged off) was changing
offsets incorrectly on the first hwvsync signal after initiating
a rate change. With HWC2.3 and prior, the correct strategy for
DispSync to employ is to enable hwvsync, and await the first observed
signal at the new rate. This patch makes the new system apply the
updated offsets at the correct time.

Fixes: b/146455831
Test: 2 new, 3 fixed unit tests
Test: boot on coral with integrated patches, fiddle with rate changes.

Change-Id: Iafae2e5112a5015441405055159538feb6c23a4b