SF: Fix thread safety for RefreshRateOverlay

Lock mStateLock when reading/writing the pointer, and writing layer
state. Destroy the layer on the main thread. Notify overlay when
viewport changes to avoid display lookup on refresh rate change.

Bug: 123715322
Test: Toggle overlay on flame.
Change-Id: I019c5cd49c94182f2c4364b0299fa3aa7783bd15