[A11y metrics] Add metrics for when a11y settings are toggled

Uses metric names that match the Accessibility.Cros* metrics
which are logged 45 seconds after startup.

Bug: b:331857972
AX-Relnotes: N/A
Test: Manually toggled all a11y settings, checked chrome://histograms.
Change-Id: Ie523ea078217ee9fd0d8eec158da0d0331a16826
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5411059


Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Nikki Moteva <moteva@google.com>
Cr-Commit-Position: refs/heads/main@{#1281609}