android {

    defaultConfig {
        applicationId = "com.nek.example" //be sure to follow the same order as here, or the id will be "null"
        buildConfigField("String", "APPLICATION_ID_DEFAULT", "\"${android.defaultConfig.applicationId}\"")
        //...
    }
}

buildTypes {
        getByName("debug") {
            applicationIdSuffix(".debug")
            //...
    }

}