flatpak: Set precise app kind from the appstream data

There can be seen runtime warnings on the gnome-software terminal like:

   Gs  Kind change on xxx from console-application to desktop-application is not OK
   Gs  Kind change on xxx from addon to runtime is not OK

They happen, because the appstream plugin sets correct kind as read from the appstream data, but then the flatpak plugin tries to overwrite that kind with the one guessed from the app ID. Make the flatpak plugin either keep already set kind or use the one stored in the appstream data, if the appstream data is available.