Add null check to a11y call.

No good will come from posting that a null View has
changed. Adding a null check so the failure will happen
when the bad call is made, rather than later when we
try to act on it in a handler.

Bug: 38122973
Test: Ran accessibility unit and cts tests.
Change-Id: I1e9aaf2a18180fcdfd0fbc0e5b716eb96446af33