SF: correct latent problem in VSyncDispatch

A negative offset calculation was incorrect in the VSyncDispatch
system. (this is code that is flagged off and in development).
Flaw was exposed in unit testing; this test corrects issue so that
the negative offset is correctly triggered.

Bug: 145013815
Test: 5 new unit tests.

Change-Id: Id8e8d0254d7875cf933675ccdb7bf864dc6c5f72