[ChromeVox] Move changed() to AutomationEditableText

As part of the effort to make the ChromeVox codebase more streamlined
and easier to navigate, we are moving towards removing redundant layers of abstraction such as ChromeVoxEditableTextBase. This change moves one function from that class to its only child, AutomationEditableText.

AX-Relnotes: n/a.
Bug: b:265481751
Change-Id: Ib816014f6c2714e7a0482e41a5ec1bd8c6707a1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5454044
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1290073}