Is there any Android app designed for Tablet?
I think you are looking for Stack Remote. There are many Android Applications for Tablets. As for a Stack Android App, Stack Remote is working perfectly on my Motorola Xoom as well as on Samsung Galaxy Note.
I have been looking for the same today, and from what I see, no current apps in Google Play seem to optimised for tablets, but couple come close:
Stack Remote : the way it shows selected stack QA sites list on to a single screen, you may find it convenient to browse it on a tablet screen.
Stack Anywhere : the answer reading is a bit annoying (since it assumes you have a small screen and shows you one answer at a time), but the rest of the interface is convenient for tablet usage.