Depend on default-dbus-session-bus | dbus-session-bus instead of dbus-x11

[Jeremy Bícha]

Debian is working to use dbus-user-session by default but allow dbus-x11 to be used as a replacement if a user wants to.

https://lists.debian.org/debian-devel/2016/08/msg00554.html

Note that gnome-session has not been ported to use dbus-user-session yet so this may be tricky to test ( https://bugs.debian.org/835887 )

Below are comments when I asked smcv about ubiquity:
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"ubiquity explicitly uses dbus-launch and so might need code changes. I am not
an Ubuntu developer, so I don't know how your installer works.

Looking at bin/ubiquity-dm, if it runs in an ordinary X11/Wayland environment
(where Xsession.d has already been sourced if using X11, and libpam-systemd
has already started `systemd --user` if it is installed), then it is probably
OK with default-dbus-session-bus | dbus-session-bus. Ideally it would trust
that its dependencies will provide a session bus, rather than
second-guessing them.

I don't know how Mir session startup works. If dbus-user-session is
installed and libpam-systemd has had a chance to start `systemd --user`,
then that case is fine. If X11 compatibility analogous to XWayland is
available and dbus-x11 is installed, then X11 autolaunch will work,
and dbus-x11 is also fine. If Mir sessions might run in the absence of
dbus-user-session and don't have X11, then the session starter
(gnome-session equivalent) might need to be prepared to launch a
dbus-daemon explicitly.

I don't know what autopilot/run_ubiquity is for (is it a test?)
but it should probably use dbus-run-session instead of dbus-launch,
like https://bugs.debian.org/836040 . Likewise
for
autopilot/ubiquity-autopilot-runner/custom-installation/iso-override/usr/local/bin/run-autopilot.sh ."