status/bluetooth: Expose adapter-state

GnomeBluetooth now exposes a more precise state than the binary
"adapter fully powerered on/off" property. Expose that on the
BtClient object to make it available to the toggle and indicator.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5773

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2444>