SurfaceView: Fix SurfaceControl synchronization issues

Hold the surface control lock in PositionUpdateListener
callbacks before checking if the SurfaceControl will
be null. UI tread might release it after the checks
causing crashes.

Also fix a case where the ViewRootImpl may be null
in the positionChanged callback.

Bug: 199261027
Test: run steps in bug
Test: go/wm-smoke
Change-Id: I5a4cac35fe14356389b29268fddf9703b25c03aa