meta-launcher: Find the current user's graphical session, not the pid's
If we're started by systemd --user, we might not be in the same session, but this is nevertheless the one we are interested in.
This is a prerequisite for the systemd --user work, where we'll move Shell to be started from a systemd unit instead of being executed by gnome-session. In that world, we are outside of the XDG session, but it's still the one we are interested in taking control of.