Convert the BrightnessController to a log scale control.

Currently, the BrightnessController's UI is a linear scale control on
top of a linear backlight control, but humans perceive brightness on a
roughly logarithmic scale. By moving to a non-linear control, we both
give users more fine-grained control over the brightness of the display
as well as a UI that works more intuitively.

Test: manual
Bug: 73810208
Change-Id: I67090ad7c4ced0420314458473c9124cb9c61906