Viewing a thread in mobile app starts the scroll controller at the first message, not the most recent

**Motivation**
The user has to scroll an enormous amount to get to the most recent message

Start at the mobile app's thread-view entry point and trace how the scroll controller is initialized when a thread opens. Reproduce with a thread containing many messages; done means opening the thread positions the view at the most recent message without requiring a long manual scroll.

Written by the indexing model from the issue text.