Accessibility serviceconfiguration via meta-data

Note: This is a part of two CL change and contains the
      system changes without updates to the settings.

1. Added a mechanism for configuring an accessibility service via
   XML file specified in a meta-data tag (similar to IMEs).

2. Added property for specifying a settings activity for an
   accessibility service.

3. Refactored the APIs in AccessibilityManager to return
   lists of AccessiblityServiceInfo instead ServiceInfo
   since the former describes an AccessibilityService in
   particular (similar to IMEs).

Change-Id: Ie8781bb7e0cdb329e583b6702a612a507367ad7b