displaymanager: Fix default session setup for GDM.

The default session setting in GDM is per user. Therefore, it is not of
much use to set it for the gdm user. (Nobody will log in as "gdm".)
Unfortunately, there seems to be no systemwide setting for the default
session, the AccountsService provides no such setting, and GDM just
hardcodes the fallback to use if it is not set in the AccountsService.
(GDM first tries "gnome", and if that is not available, it picks the
first session in the alphabet.)

Therefore, the best we can do right now is to set the default session
for the autoLoginUser. This is the case where it matters the most
anyway. We would also need the username of a non-autologin user in the
global storage to do any better.