SurfaceFlinger: allow switching when layers vote to refresh rate

This CL is a refinement of the refresh rate switching algorithm
to allow refresh rate switching even if some of the layers voted
explicitly for a refresh rate.

Test: Run ExoPlayer demo app and scroll the list while playing a video
Bug: 147516364
Change-Id: Id01ff8477804bba9e859545e20b05eeb1ec0d319