Fix a race in accessibility manager.

The accessibility manager has APIs for clients to observe changes
in accessibility, touch exploration, and high contrast states. The
notification of the listeners has to be done with no lock held but
in an attempt to do that the code was incorrectly iterating over
the copy on write collection.

bug:18840784

Change-Id: I6803ff1657fbf6b0cc7936671d5bbdebb5cbf6bb