Ensure reportFrameMetrics not being called on deleted instance

Since onSurfaceStatsAvailable gets called on binder-thread, we
need to ensure that instance doesn't get released while
onSurfaceStatsAvailable is calling reportFrameMetrics.

We do this by introducing a lock for register/unregister/callback,
such than when unregister completes, there won't be any "hanging"
callback anymore such that the callback can't be called anymore
on deleted instances.

Test: Boots
Bug: 188934435
Change-Id: I73e2d4b04cc74a152054cf7620a5c541683cb5e6