function my_acf_init() {

  acf_update_setting('google_api_key', 'xxx');
}
add_action('acf/init', 'my_acf_init');

function my_acf_google_map_api( $api ){

  $api['key'] = 'xxx';

  return $api;

}

add_filter('acf/fields/google_map/api', 'my_acf_google_map_api');