Android, AOA2, USB Isochronous Audio Streaming

Using AOA v2 protocol, a android device can output its audio stream to some accessory connected over an USB. But is it possible for the accessory to send over its audio stream to android device so that the android device will act as an USB speaker?

I'm actually planning to write a USB speaker driver using AOA protocol, but I just got stuck here. Because I can make the device initialize in AOA mode, but can't get the endpoints for audio interface. So I kind of leaning towards to believe that audio input to android device isn't possible using AOA. Anyone has any experience with that?