Back button on tablet congrats screen should return to overview

In the current tablet view, if the study options fragment is showing the congrats screen, tapping back will close AnkiDroid. This change will instead return the fragment to the overview screen, which is likely what most users would expect.

This was an attempt at fixing [Issue1488](https://code.google.com/p/ankidroid/issues/detail?id=1488), but the problem is still there.