Rename some functions for clarity.

Changing `onSpeechStopped` to `onSpeechFinished`. The callback isn't
(necessarily) used when speech is paused, but is called when speech is
finished.

Changing `playCurrentMessage` to `playTextOf(axNodeIds)`. I think it's
less confusing given that the function is called from `playNextMessage`,
`playPreviousMessage`, etc.

No functionality changes, just a minor refactor.

Bug: 1474951
Change-Id: Id418310ef2bc22134fc7ccee1a353a82482f3c48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5213415
Reviewed-by: Lauren Winston <lwinston@google.com>
Commit-Queue: Eitan Goldberger <eitang@google.com>
Cr-Commit-Position: refs/heads/main@{#1249465}