on some newer android devices, the controller buttons (of which back is one) are soft buttons that will appear in the spot where chat send arrow is. Flipping the phone 180 degrees would help, but then the button will be near the bottom quest giver sometimes. In the long run, a "use android back button to exit game" checkbox option which we could turn off would be a better answer.
Edit: and I have to say, this is an excellent example of an "improvement" that players didn't generally request.