Fix up reference times for period changes.

* When the model is reset, use the last seen reference time instead
of 0, since the reference time of 0 is guaranteed to yield inaccurate
pulses.
* When the DispSync model is updated, reduce the reference time by a
period to derisk spurious events firing when the model is updated.

Bug: 124383894
Test: systrace
Change-Id: Idb7fd5a9bacdd78133ab6451b5a79789a32e339e