connman: fix a race on the DBus interface to connman

If a service property changes between the time that the
DBus proxy for the service is created and the connection
to the "PropertyChanged" signal has been made, then this
property change is lost.

Fix this by doing an extra "GetProperties" call after
the signal connection has been made.

https://bugzilla.gnome.org/show_bug.cgi?id=770318