Make RegisteredServicesCache multi-user aware.

RegisteredServicesCache is used to track account authenticators and
sync adapters, which can vary based on user.  This change requires
that callers now provide a userId when making cache requests.  It
continues persisting into a single file for now, which is keyed based
on UID.

It now watches for package broadcasts from all users, and scans
packages on-demand.  It changes cache callers to provide a relevant
userId, and evicts cache entries when users are stopped.

Changes SyncManager to only work with accounts from running users,
only kicking off pending syncs once a user is started.

Bug: 7276595, 7316150
Change-Id: I79466a84aa69aa37e4bd9691c5d6221d3662ff29