[SurfaceFlinger] Store current offsets in VSyncModulator.

This way updated offsets are properly persisted to DispSyncSource.

Bug: 135770834
Test: systrace shows correct offsets
Change-Id: I34ed81ebecfa8a4f9826d4a7713da0ec0a8b23de
(cherry picked from commit b656aed06739e5a5ec7563970452237dca431edd)
Merged-In: I34ed81ebecfa8a4f9826d4a7713da0ec0a8b23de
2 files changed