Add pending period to DispSync.

When the period is updated due to a config change, cache the period
and don't immediately update the model until we actually observe updated
vsyncs from the hardware.

To make this more reliable, force hardware vsync to be enabled when we
first initiate a refresh rate change.

Also, override offsets with custom offsets while the period is in flux,
so that we don't fall into bad offsets when ramping up to 90hz

Bug: 128848865
Bug: 128860504
Test: systrace
Change-Id: I6aa87ad29b3effce9067a1d54d444023c7362b22