[ChromeVox] Rename AbstractEarcons to EarconInterface

Throughout the ChromeVox codebase, earcons parallels TTS and braille
naming conventions. Following this pattern, the abstract descriptor of
how earcons can be interacted with should be named EarconInterface.

This change is a pure refactor.

R=katie

AX-Relnotes: n/a.
Bug: none
Test: cq
Change-Id: I7af12600a5166a5e1bf09bf3798c3e8b7ffa197e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4032688
Reviewed-by: Katie Dektar <katie@chromium.org>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/main@{#1072968}