FCM Client only provides a way to send FCM messages & Notifications using Firebase Cloud Messaging API and doesn't collect any personal data or information. Any details you will provide stay or are saved internally within your device, and are wiped when you close, uninstall, or clear app data. However, the app sends the details as payload to FCM API, Link to Privacy Policy of FCM is given below:
Privacy and Security in Firebase
If you have any questions or suggestions, you can tell us in the Review on Google Play.