Live update memory list.

Summary:
As new memory entries are added, show them in the list right away.

- This uses the java `ArrayList` which I'd like to do away with one day.
- This moves the "Memories" header label to the layout to prevent a
  ClassCastException.

Closes T68.

Test Plan: Added a memory and saw it add to the list without restarting the app.

Reviewers: jcarter

Reviewed By: jcarter

Maniphest Tasks: T68

Differential Revision: https://phabricator.edgesysdesign.com/D71