Initially working for using Loader to handle oritation change.

Loader is a better way than AsyncTask that can handle oritation change
correctly i.e automatically destroy Activity and use a new layout file.
Fix PreviewEditActivity for rotation.
Add QuizActivity support for screen rotation.
Flushing the LearningQueue asynchronously.

Flush the LearningQueue asynchronously after refilling the queue.
So cards needed to update will not be piled up.
Main activity handles rotation correctly.
Handle the CardPlayerActivity's rotation correctly.
Fix race condition in subclasses of QACardActivity.

The race condition is caused by not cancelling all runnables for a handler
before onDestroy.
Remove hacky way to handle oritation change for QuizActivity and OptionScreen.