We try to look up the session at registration time to add wtmp
records for it.  We fail to actually find the session, though,
because we're using the "embryonic-user-session" object data
key, which is only non-NULL when the user session is still
getting setup.

This commit changes the registration code, to instead, fetch
the session straight from the manager object.

https://bugzilla.gnome.org/show_bug.cgi?id=747169