manager: Find user's current graphical session, not session of caller
If (e.g.) gnome-shell is started as a systemd --user unit, it won't be
part of the login session. We should instead look through all of the
user's sessions until we find the login session, and take that as our
session.
manager: do initial-setup post work in manager code
Right now we do the initial-setup related post work
when stopping the greeter, but the problem is we delay
stopping the greeter now until after the user session
is started.
That post-work needs to be done before the user session
is started.
This commit moves the code to a more logical place.
