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: Ifc5357bd181e0cd065cdecd0188836a35f87b3e2