AccessibilityManagerService not registered for removed packages.

The pakcage monitor in the AccessibilityManagerService was not
watching for packages that are removed. This is needes since
1) we need to remove the package from the enabled accessibility
serivces and clean up after the removed serivice; 2) we need to
disable accessibility if the last access serivices went away.

Change-Id: I06d33b411ce60703e5a2843107323ffc87046c16