- 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
91d47ccb47