react-native/local-cli/templates/HelloWorld/android/build.gradle

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

buildscript {
    repositories {
        jcenter()
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.3'

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

allprojects {
    repositories {
        mavenLocal()
        jcenter()
        maven {
            // All of React Native (JS, Obj-C sources, Android binaries) is installed from npm
            url "$rootDir/../node_modules/react-native/android"
        }
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
    }
}

ext {
    buildToolsVersion = "26.0.3"
    minSdkVersion = 16
    compileSdkVersion = 26
    targetSdkVersion = 26
    supportLibVersion = "26.1.0"
}