appdata: Install metadata to non-deprecated location

/usr/share/metainfo/ has been the preferred location for appstream
metadata since appstream 0.9.4 a year and a half ago

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#spec-component-location

https://bugzilla.gnome.org/show_bug.cgi?id=790086