Cloud Messaging Firebase не работает, в чем проблема?

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.okokokkok">
<uses-permission android:name="android.permission.INTERNET"/> //Используем интернет!
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
 
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
 
        <service android:name=".MyFireBaseInstansIDService">
            <intent-filter>
                <action android:name="com.google.firebase.INSTANCE_ID_EVENT" />
            </intent-filter>
        </service>
        <service
            android:name=".MyFirebaseMessagingService"
            android:enabled="true"
            android:exported="true">
            <intent-filter>
                <action android:name="com.google.firebase.MESSAGING_EVENT"/>
            </intent-filter>
 
        </service>
    </application>
 
</manifest>

package com.example.okokokkok;
 
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.IBinder;
import android.support.v4.app.NotificationCompat;
 
import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;
 
public class MyFirebaseMessagingService extends FirebaseMessagingService {
    public MyFirebaseMessagingService() {
    }
 
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
      super.onMessageReceived(remoteMessage);
 
        sendNotification(remoteMessage.getNotification().getBody());
    }
 
    private void sendNotification(String messageBody)
    {
        Intent intent = new Intent(this, MainActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        PendingIntent pendingIntent = PendingIntent.getActivity(this,0, intent,PendingIntent.FLAG_ONE_SHOT);
 
        Uri defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_ALARM);
 
        NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this);
        notificationBuilder.setSmallIcon(R.drawable.ic_stat_name);
        notificationBuilder.setContentTitle("okokokkok");
        notificationBuilder.setContentText(messageBody);
        notificationBuilder.setAutoCancel(true);
        notificationBuilder.setSound(defaultSoundUri);
        notificationBuilder.setContentIntent(pendingIntent);
 
 
        NotificationManager notificationManager = (NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.notify(0, notificationBuilder.build());
    }
}

package com.example.okokokkok;
 
import com.google.firebase.iid.FirebaseInstanceId;
import com.google.firebase.iid.FirebaseInstanceIdService;
 
public class MyFireBaseInstansIDService extends FirebaseInstanceIdService {
 
    @Override
    public void onTokenRefresh() {
        // Get updated InstanceID token.
        String refreshedToken = FirebaseInstanceId.getInstance().getToken();
 
        // If you want to send messages to this application instance or
        // manage this apps subscriptions on the server side, send the
        // Instance ID token to your app server.
        sendRegistrationToServer(refreshedToken);
    }
    private void sendRegistrationToServer (String tokan)
    {
 
    }
}

// Top-level build file where you can add configuration options common to all sub-projects/modules.
 
buildscript {
    
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.1.4'
        
 
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
        classpath 'com.google.gms:google-services:4.2.0'
    }
}
 
allprojects {
    repositories {
        google()
        jcenter()
    }
}
 
task clean(type: Delete) {
    delete rootProject.buildDir
}

apply plugin: 'com.android.application'
 
android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "com.example.okokokkok"
        minSdkVersion 15
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
 
dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    implementation 'com.google.firebase:firebase-messaging:17.3.4'
    implementation 'com.google.firebase:firebase-core:16.0.4'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}
 
apply plugin: 'com.google.gms.google-services'