manager: fix initial-setup -> user session transition

After the user goes through initial setup, the initial setup
program logs the user in.  Nowadays, this user session starts
on a new X server. The problem is, we also need the initial setup
X server to go away, and we need the login screen wayland server
to come up, all around the same time.

You can't quit one X server on a seat while starting another one up,
it doesn't like that, so there's caos, X server segfaults, and
unpredictable behavior as to "which session wins".

This commit fixes the problem by waiting until the greeter login screen
X server is started before starting the user session. This way they
don't stomp on each other's toes, and the user session ends up in
the foreground.

https://bugzilla.gnome.org/show_bug.cgi?id=745139