SDK Updater: Fetch source XML, display them in tree.

There's a notion of an asynchronous task with a progress bar & status UI.
That's still just experimental. Will need some refactoring, etc.

tools/sdkmanager/libs/sdklib/tests/com/android/sdklib/repository/TestSdkRepository.java

@@ -110,7 +110,7 @@ public class TestSdkRepository extends TestCase {
 
     /** Helper method that returns a validator for our XSD */
     private Validator getValidator(CaptureErrorHandler handler) throws SAXException {
-        InputStream xsdStream = SdkRepositoryConstants.getXsdStream();
+        InputStream xsdStream = SdkRepository.getXsdStream();
         SchemaFactory factory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
         Schema schema = factory.newSchema(new StreamSource(xsdStream));
         Validator validator = schema.newValidator();