Prepare the memories list view for prettyness.

Summary:
This doesn't have a hugely awesome UI aspect to it yet (in fact it shows less
than it did before - it just shows the frequency now), but with this in place
it is possible to make the ListView look really nice now.

This diff:

- Creates a `MemoryEntryAdapter` which extends `BaseAdapter` (try reimplementing
  **that** in Java and see if it's still 22 lines ;)).
- Adds an XML layout file for the new adapter's `getView()`.
- Uses `scala.collection.mutable.ArrayBuffer` instead of a Java `ArrayList` now.

Test Plan:
- Opened app in emulator
- Recalled a memory
- Saved a new memory without crash

Reviewers: jcarter, cegeland

Reviewed By: jcarter

Maniphest Tasks: T66

Differential Revision: https://phabricator.edgesysdesign.com/D81