Improve parseContent() and parseContentDepth()

The idea is that xml-roundtrip should *never* be expected from a
XmlPullParser. So what we need is a method that parses the content of an
element without relying on getText() returning text if on START_TAG or
END_TAG. This is already done by PubSubs ItemProvider.

Also add PacketParserUtils.parseElement() which will return the current
element as String and use this method in PubSub's ItemProvider.