gnome-bluetooth clears the list of devices when the adapter goes
away, but we cannot assume that that'll happen when powered down.
We don't want to show a (potentially outdated) list of devices
that cannot be interacted with in that case, so explicitly check
for the active state when returning devices.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2658>