SurfaceFlinger: Make LayerHistory virtual class

This is a base CL for refresh rate selection algorithm enhancements
by allowing a different implementation of LayerHistory which will be
controlled by a switch.

Test: watch 24fps video and observe refresh rate at 60Hz
Bug: 147516364
Change-Id: Ifd12f6683bc96cd5aba04a8960b9235729820b49
6 files changed