SF: VSyncReactor: correct distribute timestamp

Timestamp being distributed via the choreographer callback was the
anticipated vsync timestamp. This might make sense in the future, but
the correct legacy behavior was to distribute the event timestamp.

(code with problem was flagged off)

Test: correction of 2 unit tests
Test: boot to home
Test: uibench scroll, jitter not observed.

Fixes: 147487378

Change-Id: I142a6eaf849479dfe5b754db138f55d9e5870afd