Differentiate between automatic and manual brightness in metrics.

We want to make sure the trend for number of interactions with the
brightness slider goes down over time with automatic brightness enabled.
Rather than trying to correlate the setting and this metric, it's much
easier to just use a different metric.

Bug: 74610647
Test: Flash, see what gets logged
Change-Id: I9985de0c665e8d36bbe7416fe70f52098d6c2d50