SurfaceView: positionLost locking fix

positionLost can be called from CanvasContext::destroyHardwareResources
which runs asynchronously to the UI thread. This means we could be
simultaneously executing releaseSurfaces on the UI thread. We need
to expand the scope of mSurfaceControl lock in positionLost. While
we are here we add a block comment explaining the previously
undocumented locking strategy.

Bug: 156264048
Test: Existing tests pass
Change-Id: I9cdb6a0f7aeffd878f1755f240e8896f0fb8bf01