Real XML-based XPath for Android

- adds support under the '-real xpath' locator strategy
- deprecates the original xpath strategy, which will be replaced in 1.0
- because we have no way to get a dom fragment starting from an element,
  this strategy will only work for root-level queries
- this changes the xml format currently returned by `mobile: source, {type: 'xml'}`
  because that format did not include the class names as the node names, which
  is essential for using xpath properly. so if anyone is using that, it could be a
  breaking change