implement assistant service connection

Added a guest-mode to ManagedServices.
Like system services, the lifecycle of a guest is not managed.
Unlike system services, guests are not considered privledged.

The Assistant gets all the usual listener events.
Implemented adjustImportance.
Future work: enqueued, clicked, visibility, removed, annotations

Bug: 22455414
Change-Id: Ic41c0bf625b5e98cb577b49098bba23a539bb507