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
(cherry picked from commit 2cafd8b44d)