xrandr: Register our DBus API only after all other initialization

Otherwise we risk being called and not have all out internal state
ready (e.g. priv->rw_screen being NULL) leading to crashes.

We also need to disown our well known name on stop() since we'll
reaquire it if start() gets called again.

https://bugzilla.gnome.org/show_bug.cgi?id=740151