Merge changes from topic "b:140112642:4" into qt-surfaceflinger-dev

* changes:
  SF: Don't bump to PERFORMANCE refresh rate with infrequent updates
  SurfaceFlinger: DispSync: negative offsets when model is unlocked
  [SurfaceFlinger] Store current offsets in VSyncModulator.
  [SurfaceFlinger] Add vsync offset information to systrace.
  [SurfaceFlinger] Split VSyncModulator into .cpp/.h files