SF: return the expected wakeup time from VSyncDispatch

Return the expected wakeup of a callback scheduled with VSyncDispatch.
This would help the client to better plan it's work. In particular
this would be used for SF to know when to schedule the region sample
thread to capture the screen.

Bug: 181983990
Test: SF unit tests
Change-Id: I38b538c1f6fac7cadcf456c55e78f67cf637480d