1. Due to thread interleaving it was possible that
two messages are sent for requesting dispatch of
the same accessibility event and since the first
one sends the event and removes it from the pending
list the second message pulls null during the event
lookup. Look at the patch's comments for a detailed
scenario and rationale of the fix.
bug:4886129
Change-Id: If8b272ceaec7709c659ae502c3a730e63c939172
b96a41136a