Make gdm-shell-tails.desktop more similar to the one shipped in Debian's gnome-shell package

See #12364-note_33 and follow-ups. This doesn’t fix #12364 (closed) but I think we should do it anyway.

Feature Branch: bugfix/12364-greeter-races, greeter:bugfix/12364-greeter-races

Original created by @intrigeri on 12553 (Redmine)

Edited Nov 22, 2019 by intrigeri

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