kbd-backlight: Hide keyboardBrightness section when keyboard has no

backlight

* Add `HasKeyboardBacklight` to powerManagerClient and use it in
  frontend to determine whether to hide '#keyboardBrightnessSection'.

Dbus api CL: https://crrev.com/c/5402780

Bug: b/326009607
Test: browser_tests -f DevicePagePerDeviceKeyboardSubsection
Change-Id: I45baccb89c2c19d0de91fa2fc30e0497c58afd0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405792


Reviewed-by: David Padlipsky <dpad@google.com>
Commit-Queue: Longbo Wei <longbowei@google.com>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283606}