On iOS you not supposed to have Exit button, your app can be rejected if you have one. Not sure if Google will reject if you do that on Android but I would leave that out anyways. On mobile generally speaking user never really exits from an app And in fact I have never seen any app doing this so...
At one time I believe Google preferred to have an exit button, but not sure that is still the case. I believe primarily now it is there for desktop games.