IDK how NLL's ACR works, but Android API doesn't expose audio output devices because it could lead to piracy issues, such as recording whatever is playing on Spotify. I don't think there's a way to bypass that, and if there were it would probably be banned from Google Play Store.

Try ACR by NLL, it has worked great for me. Though if you change inputs in the middle of the call it keeps recording the wrong path and you end up with only half the conversation.