Crash in ViewRootImpl when accessibility is on.

We fire view subtree mutation events every X amount of time.
It was possible that the runnable that fires an event to be
scheduled more than once while this should not happen. As
a result there was a crash since we did not expect to have
a second run and nullified a reference.

bug:9422804

Change-Id: I914e163b026cd217e1536ab0ed3a6b11113c2c73