Expose APIs to analyze UIDs in UserHandle/Process.

Clients of the NetworkStatsManager public APIs may get UIDs belonging
to other users, and UIDs which don't actually represent applications.
These APIs allow clients to understand whether a given ID belongs to
the same user as themselves, and whether an ID represents an
application or something else (e.g. a system UID).

Change-Id: Ia56d5891521ce98e5594c41da8f9d756819deb66