I learned that MediaStore.Audio.Albums contains album thumbnail URIs to music files I transferred onto my phone. I also learned that these thumbnails are located in a folder called /storage/emulated/0/Android/data/com.android.providers.media/albumthumbs/ Apparently, these thumbnails have been created automatically, but not every music file has a thumbnail. 
So if the MediaStore.Audio.Albums URI value of the music file is null, is there a way to create a thumbnail in /storage.../albumthumbs/ and then set a new URI value in MediaStore.Audio.Albums`?