portal: Fix initialization order
The portal is currently first claiming the bus name, then adding the portal object+interface. This may break things with autostart as clients are able to send a message to an object path that is not there yet.
Changing the order means the object path is there when the DBus name is made known, so clients are able to talk immediately to it.
Fixes the error reported at https://github.com/flathub/org.gnome.Music/pull/24#issuecomment-702565846