MobileUpdateNotifier
For package-managed solutions that set `OSU_EXTERNAL_UPDATE_STREAM` (which overrides the config value), we should not allow the user to change the release stream themselves.
OSU_EXTERNAL_UPDATE_STREAM
UpdateManager
Valid values are `Tachyon`/`Lazer`.
This is to be used in cases where updates are handled by an external means. See https://github.com/flathub/flathub/pull/2619#issuecomment-969731305 for initial usage.