Add exclusion rect to the BrightnessDialog slider

This prevents drags that are close to the screen from triggering back
gesture. It includes the horizontal margin to give some room for
starting the drag slightly outside of the view.

Fixes: 247049806
Test: manual: small and large screen
Test: atest BrightnessDialogTest
Change-Id: I0a206eda72f6eb15a5d887f01b468a8ceba15d1f
3 files changed