Bug 951331 - [Messages][Drafts] MMS indicator missing for MMS drafts in existing (non-threadless) threads

- `thread_list_ui.js
    - set the lastMessageType to the draft's message type before applying a style to the DOM element
- Tests
    - `thread_list_ui_test.js`
        - I was not able to get the CSS style attributes to tests against - need help or tips
        - Manually tested on device