status/bluetooth: Indicate progress on toggle

Turning bluetooth on or off can be very slow ­– 10-15 seconds on
my system – and we currently don't provide any feedback that
something is indeed happening until the state changes at last.

Address this by using the `acquiring` icon when the adapter is
in a transitioning state.

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

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