build.gradle https://medium.com/@mirhoseini/how-to-setup-your-android-app-to-use-protobuf-96132340de5c

build.gradle

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    ext.kotlin_version = '1.2.10'
    ext.protobufVersion = '0.8.3'

    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "com.google.protobuf:protobuf-gradle-plugin:$protobufVersion"

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

...