shell: Use new GsApp API to handle local files

Instead of abusing the (kind == GENERIC) test, use the new
gs_app_set/get_local_file() API. Apps without appdata, such as extra
packages that we show through the org.freedesktop.PackageKit session
service can also legitimately be of the GENERIC kind and as such we
can't use it to test if it's a local file.