Make USB services multi-user aware.

USB settings are now isolated per-user, since they revolve around
installed packages.  User-specific settings are returned based on
calling user, or referenced by UserHandle passed to SystemUI.  Each
settings Context is wrapped as a specific user, so all broadcasts are
sent correctly.  Upgrades any existing USB settings to OWNER.

Physical events, like new devices, are routed to the currently active
user.  Switch to using AtomicFile when persisting settings.

Bug: 7244888
Change-Id: I8a723ad3d55ac1bff99276c5f3a3f5e8f013432f