Showing brightness slider in collapsed QS in split shade

This is first step of showing brightnesss slider - this change just makes it appear and actually working.
It doesn't cover:
- animation between collapsed slider and expanded slider
- showing slider mirror (disappearing background when changing brightness)

Bug: 193651759
Test: QuickQSBrightnessControllerTest
Change-Id: I21e71e3204352a3801386924f33094e029ac3da8