build.gradle

buildscript {
    repositories {
        google()
        jcenter()
        mavenCentral()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:3.+'
        classpath 'com.google.gms:google-services:4.3.3'
    }
}

// use postBuildExtras to make sure the plugin is applied after
// cdvPluginPostBuildExtras. Therefore if googleServices is added
// to cdvPluginPostBuildExtras somewhere else, the plugin execution
// will be skipped and project build will be successfull
ext.postBuildExtras = {
    if (project.extensions.findByName('googleServices') == null) {
        // apply plugin: 'com.google.gms.google-services'
        // class must be used instead of id(string) to be able to apply plugin from non-root gradle file
        apply plugin: com.google.gms.googleservices.GoogleServicesPlugin
    }
}