[keyboard-alt]: Add unique class to customize kb row for internal kb

- This change will allow us to reliably select the correct
  "Customize keyboard keys" row in tast tests

Bug: b/279503977
Change-Id: Id08d0db102332112a549fb4d38f38423567f6afd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4781956


Reviewed-by: David Padlipsky <dpad@google.com>
Commit-Queue: Michael Checo <michaelcheco@google.com>
Cr-Commit-Position: refs/heads/main@{#1184823}