GoogleService failed to initialize

[From Product Manager @ Google]

You can fix this issue by downloading and copying the google-services.json file for your Android app by following the steps below:

• Select your app/project name and Android packagename from this link and click Continue to Choose and configure services.
• Click Continue to Generate Configuration files.
• Download google-services.json and copy the file to the app/ or mobile/ module directory in your Android project.

If you have previously imported your Google project into Firebase, you can get the updated google-services.json from the Firebase console under Project Settings.

DO NOT COPY the PROJECT_NUMBER as suggested by one of the other answers since the google_app_id refers to your app within a Project and not the project itself.

You need to place the configuration file (google-services.json) generated by developer.google.com, as mentioned in the 2nd step of the official docs here

The process is simple

1. You can select your project or create a new one.

2. Then after selecting desired services (in this case the maps service), you can generate the configuration file.

   For people who have migrated to the Firebase projects they can get the same by going to Firebase Console, selecting your project and under settings you will find the configuration file.

3. Then as quoted in step 3 of the official docs here

   Copy the google-services.json file you just downloaded into the app/ or mobile/ directory of your Android

P.S : For people downvoting this answer, please do leave a comment as to why you are down voting it.

In my case, the cause of this error was that the Google Services plugin for Gradle and the Play Services library were incompatible versions. Instructions for compatible versions:

1) Add the dependency to your project-level build.gradle:

classpath 'com.google.gms:google-services:1.5.0-beta2'

2) Add the plugin to your app-level build.gradle:

apply plugin: 'com.google.gms.google-services'

3) If you're using Android Studio, this is the string to add to the dependency section of your application's build.gradle file:

dependencies {
  compile "com.google.android.gms:play-services:8.3.0"
}

Source: https://developers.google.com/cloud-messaging/android/client