SF: avoid dispatching close vsyncs

When the vsync period changes, the next vsync callback might be targeting
the same vsync event, that now has a newer time based on the current
period. This means that the client might be woken up for 2 frames within
the same vsync. This change tries to avoid that by making sure that the
vsync callbacks are at least a vsync period apart minus a safe distance.

Test: new unit test
Bug: 235566681
Change-Id: Ifb9b3f8b726976452d5131c8b758d1d5ca0e3639