I am using API level 27 and found a best solution after struggling for around two days. If you want to read a xml file from a class which doesn't derive from Activity or Application then do the following.
Put the testdata.xml file inside the assets directory.
Write the following code to get the testdata document parsed.
InputStream inputStream = this.getClass().getResourceAsStream("/assets/testdata.xml");
// create a new DocumentBuilderFactory
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
// use the factory to create a documentbuilder
DocumentBuilder builder = factory.newDocumentBuilder();
// create a new document from input stream
Document doc = builder.parse(inputStream);