system_api: Add SetBacklightBrightnessRequest powerd proto.

Add a SetBacklightBrightnessRequest protocol message that
Chrome will send to powerd via SetScreenBrightnessPercent
D-Bus method calls. These calls currently send raw D-Bus
arguments, but a new "cause" argument is needed.

Also add a new "MODEL" enum value to
BacklightBrightnessChange's "cause" enum.

BUG=chromium:881786
TEST=built it

Change-Id: Ife2fc6741ece56620ced37feab8e33c093aeae0a
Reviewed-on: https://chromium-review.googlesource.com/1237429


Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>