[kcms/keyboard] Restore metadata for the KDED module

Before the conversion, kcm_keyboard.desktop was used by the KCM and
keyboard.desktop by the KDED module. However, the conversion deleted both and
only created keyboard.json for the KCM, also referenced by the KDED module.

This commit brings back keyboard.desktop from before the removal, converts
it to JSON and renames it to kded_keyboard.json to avoid future mixups.