Cleanup of Card.java. Update up to Anki 2.0.23.

- Mostly formatting changes.
- Remove dead/commented out code.
- Added some asserts and renamed some methods to match libanki.
- Use separate methods for fetching simple interface content instead of modifying  libanki methods.
- Removed suspicious flag from flush() that skipped some values. This was used by our custom undo code which needs to be revisited in the future.