kbd-backlight: Use OnKeyboardBrightnessChange

* OnKeyboardBrightnessChange is an observer method that gets called by
  the browser mojo remote. This change will update the
  brightness_percent in slider whenever there is an observerable event
  related to changing keyboard brightness in the backend.

* This CL also fixes the issue where the slider element is not
  properly gated by the flag, and updates the related tests.

video: http://go/scrcast/NDY2NjY3ODg4NDQzMzkyMHxhNmQwNmU5Zi0wYg

Bug: b/326009607
Test: browser_tests -f DevicePagePerDeviceKeyboardSubsection
Change-Id: I0eb98191995360cedb8ead3bf5802e8f263d3dbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5399262


Commit-Queue: Longbo Wei <longbowei@google.com>
Reviewed-by: David Padlipsky <dpad@google.com>
Cr-Commit-Position: refs/heads/main@{#1280720}