manager: add a different hack to quit plymouth later
    
    Right now wayland sessions register with GDM before they're
    actually ready, so we quit plymouth too soon.
    
    Until we can fix that, this commit quits plymouth when the login screen
    connects to the daemon, or in the event of automatic login (where there
    is no login screen), after 20 seconds.
    
    This is like commit 862ba1bd67ec85b5784d3e8809a405f1845b1c43
    but hopefully less broken..
    
    https://bugzilla.gnome.org/show_bug.cgi?id=746498