Remove OBBs from state list when volume unmounted

Don't keep tracking OBBs when the volume they're located on goes away.
Remove them from our state tracking maps and then send a notification to
any listener that is still around.

Add a dump handler to MountService so the state of the mount lists
can be inspected.

Change the API to just make a callback directly to the change listener
when mount is called when it's already mounted or unmount called when
it's already unmounted.

Change-Id: Idb4afbb943ca5ca775825f908bff334e3ce1cfcc