Make XmlPullParser XML-ROUNDTRIP aware

xml-roundtrip is an optional feature of XmlPullParser. MXParser supports
it, while KXmlParser on Android doesn't. This means that we *must* check
if it's supported whenever we assume that XmlPullParser.getText() should
return the text for START_TAG and END_TAG. If it's not supported, like
on Android, then 'null' will be returned, which we must handle.

We also create a single method in order to obtain a new
XmlPullParser. But not all test cases do use it yet.