Ml: fix discovery-failure after onboarding skip

If storage access isn't granted, the medialibrary init will not add the internal storage as a medialibrary device. After this, it is only possible to add external devices. So later discovery of internal storage will fail, not being registered in the medialibrary database. To fix this, in the late device addition, a check for internal storage is added.

Fixes #2689 (closed)