[firebase_messaging] Updated README for Android

[oluwaseye]

Casted registry to (FlutterEngine) registry

Description

• Casted registry to (FlutterEngine) registry*

Related Issues

• GeneratedPluginRegistrant cannot be applied to PluginRegistry*

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[canberksinangil]

This would not work since required library is not imported.
import io.flutter.embedding.engine.FlutterEngine;

[canberksinangil]

@oluwaseye also how did you handle java.lang.RuntimeException: Unable to create service io.flutter.plugins.firebasemessaging.FlutterFirebaseMessagingService: java.lang.ClassCastException: io.flutter.app.FlutterPluginRegistry cannot be cast to io.flutter.embedding.engine.FlutterEngine ?