alsa: fix the legacy availability group settings

We found that the legacy availability group is not set properly for the legacy audio. See: https://gitlab.gnome.org/GNOME/libgnome-volume-control/-/merge_requests/10#note_913956 .

This is attempt to make this value configurable. We may extend the configuration scheme later, if required.

This merge request also fixes the device type for the headset microphone and alsa mixer path configuration parser (might lead to SIGSEGV).