Consolidate hex string utils

        return sb.toString();

    }

    /**

     * Convert hex string to byte array. The string length must be an even number.

     */

    @UsedForTesting

    public static byte[] hexStringToByteArray(String hexString) {

        if (TextUtils.isEmpty(hexString)) {

            return null;

        }

        final int N = hexString.length();

        if (N % 2 != 0) {

            throw new NumberFormatException("Input hex string length must be an even number."

                    + " Length = " + N);

        }

        final byte[] bytes = new byte[N / 2];

        for (int i = 0; i < N; i += 2) {

            bytes[i / 2] = (byte) ((Character.digit(hexString.charAt(i), 16) << 4)