liblights: add support for adjusting notification LED brightness

The old exynos4x12 lights HAL had logic for scaling the brightness of
lights according to the light's type and also scaling each R/G/B colour.

Bring this logic across, and make the battery LED not eye-searing at
night.

Based on work by Daniel Hillenbrand.

REGRESSION-66

Change-Id: Ia6108468f57c3b6c6a1a68da76a1c3362fd982a5