Move XML object factory logic to libcore

It makes sense to hide the details of the parser implementation
(and other similar objects) in libcore so it could be changed.

This change removes the "ExpatPerformanceTest" which appears not to have
been maintained and was comparing KxmlParser with itself. It is assumed
that android.util.Xml used to return the expat parser.

Test: build
Bug: 111055375
Change-Id: Ibad247323ba90cd949aecb2bd92f2f73306a4327