Use the same model for downloaded items, topic tree models, and search results

- Fixes back navigation for downloaded items
- This diff also fixes initialization
- This diff also moves topic tree data get into models and sets models
  explicitly depending upon KA.

Test Plan:
- Load a dowanloaded video and make sure it shows as downloaded
- Navigate to the same item and make sure it shows as downloaded
- Ditto for search.
- In all cases, make sure back goes to the parent topic.  No matter
  where you were.