Finish BrightnessDialog when shade is opened.

This ensures that BrightnessDialog won't appear in the background when
controlling the brightness from QS.
Also removes the unused lazy ShadeInteractor from the test.

Fixes: 243630115
Test: Manually, opening both brightness sliders at the same time
Test: Added unit test
Flag: NONE
Change-Id: I4d1a62fcd87ac4836df9c4a5bb0648b4eced47cc