"Dimmed" brightness level should be a factor of current "bright" brightness

Steps to repro:
- enable auto brightness
- go to a dark room
- leave the phone with screen on for a minute

Expected:
- a few moments before going off, brightness is reduced

Current:
- a few moments before going off, brightness is increased to some static "dim" level

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: unity-system-compositor 0.0.5+15.04.20141109-0ubuntu1
Uname: Linux 3.4.67 armv7l
ApportVersion: 2.14.7-0ubuntu10
Architecture: armhf
Date: Tue Nov 25 16:51:47 2014
GraphicsCard:

InstallationDate: Installed on 2014-11-25 (0 days ago)
InstallationMedia: Ubuntu Vivid Vervet (development branch) - armhf (20141125-020205)
SourcePackage: unity-system-compositor
UpgradeStatus: No upgrade log present (probably fresh install)
version.libdrm: libdrm2 2.4.58-2
version.lightdm: lightdm 1.13.0-0ubuntu2
version.mesa: libegl1-mesa-dev N/A