connections: Listen on NMDevice::state-changed instead of notify::state

We need to be sure that NMDevice's properties are already up to date, as we're
accessing the active_connection from withing the signal handler.

https://bugzilla.redhat.com/show_bug.cgi?id=1149200
connections: Don't give up if we've not seen an active connection yet

It will appear later on.

https://bugzilla.redhat.com/show_bug.cgi?id=1149200
connections: The active connection state might already have progressed

Trigger the signal handler once when we hook it.

https://bugzilla.redhat.com/show_bug.cgi?id=1149200