Bug 906581 - [Messages] Keyboard should always close when pressing Send

https://bugzilla.mozilla.org/show_bug.cgi?id=906581

- Don't execute transitionend handler for recipient list visibility unless app is in "new" view
- Don't manually refocus
- Adds conditional guards to avoid TypeErrors when placeholders are null
- Tests
    - Switching multiline to singleline
    - Switching multiline to singleline + refocus
    - Switching singleline to multiline

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>