Fix ringer drawer touchable area

Use ringer drawer background as a touchable area as it aligns closely to
the buttons while the container is matching its parent to allow enough
space for the animation

Flag: com.android.systemui.volume_redesign
Fixes: 394086402
Test: manual on the phone
Change-Id: Ic2a45700c8817cc405e26b6e78b8267c246cf068