플러터에서 앱을 종료하는 것은 아래 방법을 씀.
static Future<void> pop({bool? animated}) async {
await SystemChannels.platform.invokeMethod<void>('SystemNavigator.pop', animated);
}
출처 :: https://api.flutter.dev/flutter/services/SystemNavigator/pop.html
관련된 stackoverflow ::
https://stackoverflow.com/questions/45109557/flutter-how-to-programmatically-exit-the-app