Fatal system crash due to NullPointerException

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