XmlUtils: Fix hex parsing in readThisByteArrayXml

hex 'a' is not correctly deserialized in readThisByteArrayXml.

Change readThisByteArrayXml & writeByteArrayXml to use HexEncoding for
hex conversions.

Bug: 138482990
Test: Was seeing parsing errors with ag/9135251.
Validated the fix with ag/9135251.

Change-Id: If9f86b5ed85a9f883130e4110f85cd2faa995439