Scroll the deck list to the currently selected deck and center it

On tablets, the currently selected deck is highlighted in the deck list. If there are many decks, then the selected deck may not be initially visible. This change will scroll the decklist to ensure AnkiDroid always opens up on tablets with the currently selected deck visible.
Merge pull request #348 from hssm/center-selected

Scroll the deck list to the currently selected deck and center it