You can use WebClient DownloadStringAsync and parse the result into an XDocument. Based on the XML you provided, it seems like you can use a GraphicsLayer with UniqueValueRenderer (Attribute="precinct") that uses SimpleFillSymbol. Your UniqueValueRendererInfos will have a different color per precinct value. By setting the renderer, you do not have to create a symbol on individual graphics. But you will still need to create each graphic based on the ward information you just parsed so that each ward becomes a graphic in your GraphicsLayer with geometry from the given pointcollection, and attributes from the given "name" and "precinct" values.
You can look at our SDK samples for guide: esriurl.com/slsdk2.
Good luck.