Starting point for User Restrictions API

Restrictions saved as key/value pairs, mostly booleans right now
but might be expanded to other types later.

Save and restore restrictions in the user manager service.
Enforce some of the restrictions at the framework level. Some
are enforced (also) at the app level, such as in Settings.

Change-Id: Id11ffe129cb6a177e094edf79635727388c26f40