UrfArbitrator: handle flight-mode: no-adapter

While setting all devices in set_flight_mode(),
if a particular device is in no-adapter state,
then it can't be manipulated, but it's desired
state can be saved.
src: fix property state change signals

This patch was authored by Antti Kaijanmaki <antti.kaijanmaki@canonical.com>.

It fixes signal state changes which fix problems being seen
by the new flight-mode UI.