SurfaceFlinger: VSyncReactor should not know VSyncDispatch

VSyncReactor doesn't use VSyncDispatch anymore with the exception
of logging the predicted vsync. Decouple those two for better logic
separation.

Test: SF unit tests
Test: examine systraces with debug.sf.show_predicted_vsync=1
Bug: 162888874

Change-Id: I368926f55a21663cdea478219bf467fc4772aadb