mixer-control: Fix network devices not having icons
Fix Airplay devices not having icons in gnome-shell and gnome-control-center.
Note that it would be pretty cool if we had a better icon than audio-speakers
that Pipewire always uses for Airplay devices:
https://github.com/PipeWire/pipewire/blob/master/src/modules/module-raop-sink.c#L1896
CC: @jimmac