Flutter App for Firebase Alerts

I need a simple Flutter application that plays a loud sound and displays an overlay when there's a change in Firebase Realtime Database.

Requirements:
- Must work:
- Even when the app is closed
- Even when the device is locked
- Even when the device is rebooted

- Platform: Android
- No special design needed; I’ll provide the overlay design.

Ideal Skills & Experience:
- Experience with Flutter and Firebase
- Knowledge in background services for Android
- Ability to complete the project today

Freelancers with similar experience (like calling apps) are preferred.