[flutter_local_notifications] Add FlutterLocalNotificationsPlugin.setConfigureFlutterEngineCallback for Android

[alexmercerind]

Summary

If a AndroidNotificationAction with showsUserInterface: false is clicked, a new Flutter engine is created by the plugin internally. I have some platform channels inside my MainActivity, which I want to be accessible inside this newly created engine as well. #2338 describes a similar situation.

Thus, something similar to following can be used to configure the newly created Flutter engine e.g. registering a new platform channel:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    FlutterLocalNotificationsPlugin.setConfigureFlutterEngineCallback {
        configureFlutterEngine(it)
    }
}

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    FlutterLocalNotificationsPlugin.setConfigureFlutterEngineCallback(::configureFlutterEngine)
}