Add UserRecords to separate user interactions

Each user record maintains the list of sessions and providers that
are running under that user. Lifecycle for providers has been modified
to stop discovery when the user is no longer current but keep the
binder connection open so long as there's a session that has selected
a route from that provider. When a user is stopped all providers on
that user will be unbound even if they were still in use.

Change-Id: Iadf1efded3415f7ecf384d3a73513883de9c86b0