SurfaceFlinger: stabilize heuristic refresh rate calculation

When calculating layer's refresh rate heuristically, also store
a history of the last refresh rate calculated. We use this list to
determine how consistent is the refresh rate. If the refresh rate is
not consistent, then we keep reporting the last consistent refresh rate
and wait for the refresh rate to stabilize again.

Test: Play at 60fps video in XPlayer
Bug: 157540021

Change-Id: If3b525820d298cc5835dddf73f327501c8a18964