[Backport to 42] kbdA11yDialog: Use MetaKeyboardA11yFlags
The change in mutter to move keyboard accessibility into backends needs to be applied in gnome-shell as well, otherwise the keyboard accessibility dialog cannot work.
Closes: mutter#2306 (closed) Part-of: !2334 (merged) (cherry picked from commit ed910a72)
Edited by Olivier Fourdan