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
cc4053e031