React-Navigation for know if the root screen. Let us manages back button android.

isRootScreen.js

// @flow

export function isRootScreen(navigator: {
  index: ?number,
  routes: ?Array<Object>,
}) {
  if (navigator.index == null) {
    return true;
  }

  if (navigator.index > 0) {
    return false;
  }

  return (
    !navigator.routes || !navigator.routes.find(route => !isRootScreen(route))
  );
}