bluetooth: Set up hardware gain control if init volume is received late

Originally written for A2DP this rework of that patch enables late-bound
hardware volume control on HFP and HSP.  As per the specification the
headphones (where gain control for both speaker and microphone could
happen in hardware on the peer) are supposed to send initial values for
these before the SCO connection is created; these `AT+VG[MS]` commands
are also used to determine support for it.  PA uses this information in
`add_{sink,source}` to attach hardware volume callbacks, _if_ it is
supported.  Otherwise PA performs the attenuation in software.

Unfortunately headphones like the WH-1000XM3's connect to A2DP
initially and only send `AT+VGS` (microphone hardware gain is not
supported) _during_ SCO connection when the user switches to the HFP
profile afterwards; the callbacks set up dynamically in
`rfcomm_io_callback` are written after the sink and source have been
created (`add_{sink,source}`), leaving them without hardware volume
callbacks and with software volume when adjusted on the PA side.  (The
headphones can still send volume updates resulting in abrupt changes if
software and peer volume differ.  Furthermore the same attenuation is
applied twice - once in PA software, once on the peer).

To solve this problem we simply check whether the callbacks have been
attached whenever the peer sends a volume change, and if not attach the
callbacks to the sink/source and reset software volume.

Fixes: d510ddc7 ("bluetooth: Perform software attenuation until HF/HS reports gain control")
Part-of: <pulseaudio/pulseaudio!528>
2 jobs for late-bound-hardware-volume in 4 minutes and 3 seconds