Hi,
Android users are used to Google-style BACK button behavior, which is to go back one screen, not to close the app. So it is very frustrating to learn special habbits only for your app and be carecull to not press BACK button, because this will close the app.
So, my suggestion is BACK button should only close the app once in FIRST screen (nothing selected, no route, no nav going on), but not on any other. On other screens BACK should go back one screen only.