fix memory leak due to statically declared DeckTask variable

`DeckTask sLatestInstance` in Async.DeckTask gets stored statically so that we can keep track of the previous task, however this was causing a memory leak, which became dramatically enhanced when the whiteboard is enabled, especially on devices with large screens -- [see issue 1907](https://code.google.com/p/ankidroid/issues/detail?id=1907).

This fixes the problem by destroying the reference to the old `DeckTask` object once the new `DeckTask` has finished.