Making mirror slider appear when brightness slider is used in split shade

This change is mostly about QuickQSPanelController which should have very similar brightness mirror logic to QSPanelController.
They have common base class QSPanelControllerBase which could be used for sharing the logic, but:
- brightness slider logic differs a bit and it would be cumbersome to share brightness slider logic in base class
- from above, it would be weird to have brightness mirror logic in base class, but not brightness logic
- favour composition over inheritance

That's why brightness mirror logic was extracted to BrightnessMirrorHandler and a few duplicated lines remain.

Bug: 193651759
Test: QuickQSBrightnessControllerTest
Change-Id: Iea604092cd84f1da96cdbc5cd085babc0dc2bb49