rfkill: Use promise to initialize proxy

rfkill: Use promise to initialize proxy

There's no good reason for waiting for the proxy to be initialized
to connect signals. In fact, connecting the signal beforehand
ensures that the handler is in place when the proxy fetches the
properties, so we don't have to call the handler explicitly.

That in turn will allow us in a follow-up to rely on the signal
parameters to only process changed properties.

To achieve that by constructing the proxy manually, and then
initialize it asynchronously in a Promise.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2385>