manager: don't connect to systemd until exported on bus

We now properly resync our active state after being exported
over the bus, but we still have a signal connection set up
prematurely.  If it did get called, it would lead to scary
messages on the console.

This commit moves the signal connection down to the same place
we test our active state at startup.

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