I'm a little bit confused by your question. You mention you want to get the root of your app, but then you point to the data storage directory. The root of your app should be at cordova.file.applicationDirectory + "www/". These will resolve to the locations where your app source files are located on each device platform. I believe this is what you need.
Regarding the data files folder - this is a data storage folder that is used for downloading files needed for your app. To the best of my knowledge there is no such thing as app GUID. All mobile apps are identified by their package ids. In your case this is com.telerik.TSTEST. The GUID that you see in this folder is most probably generated by a plugin that needed to download some files. Can you please explain why you think this is the root of your app?
I'm looking forward to your reply.