metadata: Add form factors to make kcm appear on mobile again

Summary: Seems to be required to make the kcm visible in plasma-settings

Reviewers: #plasma:_mobile, #plasma, nicolasfella, bshah

Reviewed By: #plasma:_mobile, #plasma, nicolasfella, bshah

Subscribers: bshah, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24451