A few improvements around the DeckTask.

- Made fields final and added a constructor.
- Remove static sOldInstance and instead store a mPreviousInstance in
  the task itself. This also guaratees the correct sequencing of tasks.
- Rename static sInstance to sLatestInstance to reflect the semantics.
Merge pull request #28 from flerda/decktask2

A few improvements around the DeckTask.