Fix Volume Dialog dismiss on touch outside

This change reworks how Volume Dialog registers the touchable views to
determine dialog area. This allows the ringer drawer, which occupies the
whole top of the dialog, to only register the actual drawer.

Flag: com.android.systemui.volume_redesign
Fixes: 394086402
Test: manual on the phone. Open Volume Dialog and try to tap outside of it
Test: atest VolumeDialogScreenshotTest
Change-Id: Ib05f7dfa950708b434c6b3a35aa9659a94cbdca1