Cleaning up MessageListFragment.

- Introduced EmptyCallback to avoid the flood of null checks.

- Fixed some TODOs in MessageListFragment.
  - Notably, sendPendingMessages() is now handled by the fragment itself,
    rather than by the activity.
  - Moved two DB accesss from the UI thread to a worker thread.

- Replaced the 'mailboxid < 0' check with isMagicMailbox(), which is more
  explicit and easier to understand.

- Renamed some methods in preparation of moving to the activity.

Change-Id: Ie730c2c561050bbfa83a38252fcf09d59238f7ea
4 files changed