Fix nuxeo-data-list arrow key navigation highlighting 2 docs

When using arrow keys to navigate up and down a nuxeo-data-list element, two Documents appear to be selected simultaneously (not including the first and last entries).

Steps to reproduce:

1. In Web UI, navigate to a drawer tab that contains a nuxeo-data-list element (e.g. Tasks, Favorites, Collections, etc.)
2. Select a Document besides the first or last item using a mouse click
3. Press either the up or down arrow key to navigate to a different Document

Expected result: the current Document navigated to is highlighted with a blue line

Actual result: both the current and previous Document are highlighted with blue lines. This repeats when moving up or down further along the nuxeo-data-list using the arrow keys.

Note: selecting a Document with the mouse following this results in just one documented being highlighted as expected.