Add a service to my android app to get notifications whenever a fire base child is added

Question

I am trying to create an android app where a notification appears whenever a firebase child is added to my JSON, I am able to get notifications only when I open the app, or when I close it recently. I am having some issues with it. 1) Whenever newly installed, I get notifications for all the child items that were previously added. I only want the freshly added child notifications. 2) I want to get notifications even when the app is not running, i.e, in background. Can someone please help me create a service for this, so that I can get this working.

Please also help me out with the below code, if I am doing something that doesn't make sense.

package com.fayaz.firebasenotify;

import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Intent;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.app.NotificationCompat;
import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import java.util.Random;

public class MainActivity extends AppCompatActivity {

FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference myRef = database.getReference("user");

NotificationCompat.Builder builder = new NotificationCompat.Builder(this);
int notify=1;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Intent notificationIntent = new Intent(this, MainActivity.class);

    final PendingIntent contentIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);

    // Read from the database
    myRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            String value = dataSnapshot.getValue(String.class);
            String name = dataSnapshot.getKey();
            Uri alarmSound = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
            builder.setSmallIcon(R.mipmap.ic_launcher);
            builder.setContentTitle(name);
            builder.setContentText(value);
            builder.setStyle(new NotificationCompat.BigTextStyle().bigText(value));
            builder.setContentIntent(contentIntent);
            builder.setSound(alarmSound);
            builder.setAutoCancel(true);
            notify=generateRandom();
            NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
            notificationManager.notify(notify, builder.build());
        }

        public int generateRandom(){
            Random random = new Random();
            return random.nextInt(9999 - 1000) + 1000;
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}
}