Crash in the touch explorer.

1. The touch explorer was notified for accessibility events from
   a binder thread which was poking the internal state of the
   latter which by design is not tread safe. Since the touch
   explorer is expected to be running only on the main thread
   the accessibility manager service delivers the accessibility
   events to the explorer on that thread.

bug:6635496

Change-Id: Ifdc5329e4be8e485d7f77f0fb472184494fa0d15