I just checked, unless I'm missing something there isn't anything in the Android App that is labeled USB redirection. I tried a few different models that we have here, Dell Chromebook 5190 is what most of our students have.
Plugging in a flash drive, the Chromebook sees the device, but Horizon View doesn't.
Is there something specific I need to enable in the client? It's 4.1.0-11281671