Can't find which lib asks for READ_EXTERNAL_STORAGE (Android) - private photos etc

Editor version: Unity 5.4.2p3. (Android) Test device OS: Android 6.0.1

I'm really frustrated by an alert window showing up on game launch and asking for permission with message "Allow Game to access photos, media and files on your device?". Looked through all manifests in my project and haven't found any manifest that contains any of these lines:

 <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
 <uses-permission android:name="android.permission-group.STORAGE"/>

If it isn't Unity, how I can track down the third party lib that requires this dangerous permission without going into removing them one by one?

UPDATE: when I set Write Permission from External to Internal the problem goes away. However I won't be able to save screenshots for sharing. Is it normal behaviour for Unity?