Fixing brightness slider jank in split shade

This jank was caused by two sliders - one in QS and one in QQS - being updated at the same time.
When sliding one, the other one register changes in brightness and is trying to animate itself into the new value. Because they share one mirror, seems like mirror is getting conflicting signals about current values - one mirror animates, the other doesn't.
Another issue this fixes is changing brightness significantly and quickly switching to collapsed or expanded QS - the animation would not end till new slider shows up and slider would be moving, even though the other slider was not moving.
Also renaming BrightnessSlider to BrightnessSliderController (because it seems to be naming convention for controllers) - that makes this change so wide-reaching.

Fixes: 196523321
Test: open split shade -> slide finger (not tap) on top of slider to change brightness -> lift finger and see no jank
Change-Id: Ie11e3670f1464e5f1c85e221ce09b6ebf6dd4d0c