SF: add partial implementation of VSyncReactor

Adds an adaptor for the VSyncDispatch/VSyncTracker system
that will implemente the DispSync interface. Implementation
is only partial, will build out rest in subsequent MPs.

Bug: 140303479
Test: 6 new unit tests (VSyncReactor)

Change-Id: I11441f0f5bac9095d14122f487ab99bbdf44a7e3