SF: more DispSync improvements.

Pass the reference time to DispSyncThread. Since the phase offset is calculated
using timestamps relative to the reference time, we must also adjust the phase
offset by the same reference time when computing the next refresh time.

Always reset phase offset to zero when updating the reference time because the
reference time equals the first timestamp.

After beginResync() we need to keep HW vsync enabled until the model is updated.

Bug: 25113115
Change-Id: I8eae227bee91c24a99bf8e57fbebceb98d29c77d
Test: check in systrace that app/sf vsync events have correct phase