Move XmlDocumentUtils.writeValueByType()

Now that we're using XMLStreamWriter, the only user lives in DomUtils,
so let's move the functionality over there.

Change-Id: Ia3cb69f5222ffb9c645bc91f79d1aa908b4fe971
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Move XmlDocumentUtils.writeValueByType()"