Brightness slider doesn't jump when panel is open

Slider starts listening and is set to current value as soon as QSPanel
starts opening.

Slider animation duration is proportional to change value.

QSPanel decides when to listen based on QSFragment listening and
expanded variables.

Test: manual
Change-Id: Ie310da4942b57d8016da0986071445b1cd3349df
Fixes: 78209596