Comment by tadfisher

Define "storage backend". You can implement a FileProvider to plug into the Storage Access Framework as a provider, so other apps can browse and request permissions for all of your app's files. You can use the Storage Access Framework as a consumer and ask the user for permissions to read any directory on external storage that is not Downloads/ or Android/{data,obb}, and any files exposed by other apps. Google's own apps use these APIs and do not declare MANAGE_EXTERNAL_STORAGE.