Add metrics to brightness slider

Had to add a way for BrightnessController to know when its the end
of a touch, so that we don't spam the event logs with intermediate
values.

Added visibility to BrightnessDialog as this is what settings
launches.

Bug: 21528168
Change-Id: Ie214b4ddb0c9f9bbe8c4f182f9c59f229963ebc7