g-s-d: Provide kill switch state
gnome-settings-daemon should provide the state of the hardware kill switches via org.gnome.SettingsDaemon rfkill (or similar). Applications like calls and phosh can then just query it.
This should abstract away the case where the device is actually "unplugged" and not visible to the OS anymore. The short hack is to hardcode the devices. In the long run we might be able to get them from DT and compare to the devices we see to determine which ones are currently off (which would allow upstream merging).