manager: find session at registration time
    
    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