[Flexiglass] Don't use padding for BrightnessMirror

Instead, just center it. The composable will handle it's own size to
match the real slider and any padding necessary.

Test: atest PlatformScenarioTests:SwipeBrightness
Test: manual, large screen portrait
Fixes: 372754645
Flag: com.android.systemui.scene_container
Change-Id: I790b392741fe5cab4e3deaaedf21c918af6372f2