bluetooth: Update source software volume on AVRCP SetAbsoluteVolume

bluetooth: Update source software volume on AVRCP SetAbsoluteVolume

The A2DP spec mandates that the audio rendering device - the device
receiving audio, in our case a `pa_source` - is responsible for
performing attenuation:

AVRCP v1.6.2, §5.8:
    The SetAbsoluteVolume command is used to set an absolute volume to be used by the rendering device.

BlueZ models this call as a change of the `Volume` property on the
`org.bluez.MediaTransport1` interface.  Supporting Absolute Volume is
optional but BlueZ unconditionally reports feature category 2 in its
profile, mandating support.  Hence remote devices (ie. a phone) playing
back audio to a machine running PulseAudio assume volume is to be
changed through SetAbsoluteVolume, without performing any local
attenuation.

Future changes will implement this feature the other way around: setting
an initial value for the `Volume` property as well as propagating
`pa_source` volume changes back to the peer.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/239>