queue2: Handle buffering levels on NOT_LINKED

queue2: Handle buffering levels on NOT_LINKED

When downstream returns NOT_LINKED, we return the buffering level
as being 100%.

Since the queue is no longer being consumed/used downstream, we
want applications to essentially "ignore" this queue for buffering
purposes.

If other streams are still being used, those stream buffering levels
will be used. If none are used, upstream will post an error message
on the bus indicating no streams are used.

https://bugzilla.gnome.org/show_bug.cgi?id=785799