Improvements to Restrictions API

Convert restrictions provider to a service instead of a receiver.
Add a way to get pending responses from restrictions provider.
Add AbstractRestrictionsProvider.
Add a callback API for responses.

Removed some constants in RestrictionsManager.
Added new constants for errors and error codes.
Much improved javadocs.

Bug: 16176009

Change-Id: I838a50fabc80b94b632294b3a55cd5d8092acf55