Remove margin between ANC buttons

Remove the 4dp margin between buttons in ANC Slice to make it consistent with design. It should be safe to override it because there is no other SliceView usage in SystemUI.

Test: m SystemUI
Flag: ACONFIG com.android.systemui.new_volume_panel TEAMFOOD
Bug: 322279959
Change-Id: I93dee1700259604299d56e4cacae82023e0de15d