Many content providers/publishers still restrict their protected content to iOS-only devices since it's a highly protected system and they don't want people to be able to (for example) copy or re-stream their content. With Android (being an open system) that's very easy and protected content can't be 100% protected.
Presumably, this is from the publisher of the audiobook versions of the textbook material (and probably not your university itself).
The only option would be to get an iPad, iPhone, or (most probably) an iPod Touch. Personally I would see what their requirements are and pick up a used older device that still meets their iOS requirements if you want the convenience of the audiobook.
It's possible your university may have devices you can check out for this purpose without having to buy one. I would definitely suggest asking the IT department and the library if they have those available to students.