private static final String firexFox_userAgent = "Mozilla/5.0 (Windows NT 6.1; rv:40.0) Gecko/20100101 Firefox/40.0";

private static final HttpRequestInitializer intializer = new HttpRequestInitializer() {
    @Override
    public void initialize(HttpRequest request) throws IOException {
        final HttpHeaders headers = request.getHeaders();
        List<String> agents = new ArrayList<>();
        agents.add(firexFox_userAgent);
        headers.put("User-Agent", agents);
    }
};

private static UserRegistrationApi mUserRegApi mUserRegBuilder = new UserRegistrationApi.Builder(EnpointConsts.sAndroidHttp, EnpointConsts.sGsonFactory,            CredentialHolder.getCredential()).setApplicationName(Consts.APP_ID).setHttpRequestInitializer(intializer);