system_api: Add power_manager::kSetScreenBrightnessMethod.

Add a new "SetScreenBrightness" constant. The existing
powerd SetScreenBrightnessPercent is being updated to take a
protocol message instead of D-Bus args, so we might as well
rename it to be shorter at the same time.

BUG=chromium:881786
TEST=used with updated powerd

Change-Id: I720b259fe518e0a9a45f981a0ba1db68d9a726b4
Reviewed-on: https://chromium-review.googlesource.com/1242034


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>