Previously it was a specific case, now introspection is just another
interface, which is always implemented.  It is registered/unregistered
when an object path is referenced first/last.

When a paired device is removed an unnecessary PropertyChanged signal is being
emitted in the device path that was removed previously.

Since src/bluetoothd needs libbluetooth, add it to its dependencies.
Parallel build tested with make -j16.

The headset_suspend_stream function should imediately fail if the headset
isn't connected or playing. This ensures that we return a proper error to
the unix socket client if the headset disconnect just before we receive a
STOP_STREAM request.

We should reject any suspend request if we've already started the close
procedure for a stream.

Since we don't track SCO handles the most common reason for this message
is a disconnected SCO link, i.e. it's not an error as such.

The media stream could be gone by the time that the AVDTP command timeout
expires (e.g. if the media transport channel gets closed prematurely). In
such a case there's no need to send a separate ABORT command but we can
directly proceed with closing the signalling channel.

The transition to ABORTING state when acting as initiator should happen
when we receive the ABORT reply and not when we send the initial ABORT
command. The reply handling is already correct but since there's a state
change also in the sending part the later state change request would fail.
This patch fixes the issue by removing the state change when sending the
ABORT command.