Detect app killed and cleanup Firestore. Voice chat app made by Flutter

I'm making a voice chat app using Flutter for iOS and Android. I use Firebase Firestore to keep active users in a chat room. When a user kills an app (by swiping up in app list), I have no way to detect it. So there is no chance to remove a user from Firestore users field. I'd like to keep a user in firestore while an app is background. How to successfully know when a user leaves an app? Btw, I'm using Agora RTC SDK for voice call. Thank you for reading.

Firestore has the below structure

chatRoom (collection)
  - docidA (document)
    - users: ["name1", "name2"] (Array field) 

Source code:

class RoomPage extends StatefulWidget {
  const RoomPage({Key key,}) : super(key: key);
  @override
  _RoomPageState createState() => _RoomPageState();
}

class _RoomPageState extends State<RoomPage> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    // Join into a chat and add myself to firebase firestore users field
    addMyselfToFirestore("docidA", "name1");
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {  // Unfortunately Not called when a user kills this app.
    super.dispose();
    removeMyselfFromFirestore("docidA", "name1");
    WidgetsBinding.instance.removeObserver(this);
  }

  @override
  build(BuildContext context) {...}

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    switch (state) {
      // Unfortunately detached not called when a user kills the app
      case AppLifecycleState.detached:
        removeMyselfFromFirestore("docidA", "name1");
        break;
    }
  }
}

Flutter

$ flutter --version
Flutter 2.0.4 • channel stable • https://github.com/flutter/flutter.git
Framework • revision b1395592de (7 days ago) • 2021-04-01 14:25:01 -0700
Engine • revision 2dce47073a
Tools • Dart 2.12.2