bluetooth: Refactor dependency to org.bluez.Audio

bluetooth: Refactor dependency to org.bluez.Audio

The state of this interface is needed for one single reason: we need to
wait until all profiles have been connected (or more precisely, until
are connection attempts are finished). This can be made more explicit in
the code by just checking the CONNECTING state (and not loading
module-bluetooth-device during that state), but otherwise treating all
transport types equally.

Ideally, audio_state should be completely removed but it's left there to
avoid an issue with module-card-restore, as documented in the source
code's comments.