Add support for getting the resourceId attribute of an element

It makes a lot of sense in our tests to get access to the resource id of an element. This pull requests add support for getting it through the getAtrribute api.
There's a [helper for this](https://github.com/appium/appium/blob/master/lib/devices/android/bootstrap/src/com/android/uiautomator/common/ReflectionUtils.java#L64) in ReflectionUtils.