Fix SessionIsActive property

GsmSystemd emits its cold-plug notify::active signal out of
init(), giving GsmManager no chance to connect its signal handler
in time to catch the initial emission. And even if we did,
the GsmManager handler can only operate once the skeleton object
exists.
Therefore, do our own cold-plug after creating the skeleton.

This is fallout from the GDBus port.

https://bugzilla.gnome.org/show_bug.cgi?id=745762