The file-list now uses native event listening to handle `enter`
keypresses (see Issue 7277 for more context). Because gr-button still
used iron-a11y-keys bindings, the `enter` event that was caught and
cancelled by the button was different from the `enter` event that was
caught by the file-list.
Migrating to native event listening on gr-buttons allows the event to be
properly cancelled.
Bug: Issue 7470
Change-Id: I7998243ce16b5cb5e6d3f903f184d39d89a1090d
(cherry picked from commit 2de6749191)