kbd-backlight: Implement KeyboardAmbientLightSensorEnabledChanged in powerManager

* KeyboardAmbientLightSensorEnabledChanged is called when the keyboard
  ambient light sensor status changes, allowing us to observe the ALS
  status change and update accordingly.

BUG=b:326009607
Test= chromeos_unittests -f PowerManagerClientTest

Change-Id: Ie637b92c6dc98118a0001fe78705a7a8c4676e96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5547966
Commit-Queue: Longbo Wei <longbowei@google.com>
Reviewed-by: David Padlipsky <dpad@google.com>
Cr-Commit-Position: refs/heads/main@{#1304648}