status/bluetooth: Use promise to initialize proxy

status/bluetooth: 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 allows us to rely on the signal parameters to only process
changed properties.

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

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2391>