Refactor BrightnessDialog layout calculations

Simplifies the usage of the `margin` variable within the `setBrightnessDialogViewAttributes` method. Since this variable was always initialized to 0 and never modified, its use in width calculations and for setting gesture exclusion rectangle bounds was effectively a no-op.

Flag: EXEMPT - just a small refactor
Bug: None
Change-Id: I7d66c1ae27aa92e43b597ce2c78b95e18feb5091