Bug 959661 - [Messages][Drafts] Only set MessageManager draft if not in thread (to thread's draft, or null if thread has no draft)

- `message_manager.js`
    - only call `fromDraft` if the thread has a draft
    - set `MessageManager.draft` to null if thread doesn't have a draft
- Tests
    - `message_manager_test.js`
        - update test to only verify that `fromDraft` is called if thread has a draft
        - manual tests on device