data: use a11y menu in initial-setup session
The a11yGreeter menu was removed in gnome-shell 3.27.1. It was identical to the a11y menu, except that a11yGreeter was unconditionally visible, whereas a11y hides itself unless a a11y tool is enabled or the always-show-universal-access-status GSetting is true.
Using the correct name means the a11y menu will be shown if the distro or administrator has overridden always-show-universal-access-status, but is not enough to unconditionally show it.