Make Volume Dialog not cancellable to disable back gesture

Flag: com.android.systemui.volume_redesign
Test: atest VolumeDialogScreenshotTest
Test: manual on foldable. Open Volume Dialog and try to perform a back
gesture
Fixes: 395818644

Change-Id: I7a242b5619252799c6fdbf86c575a6e2bd88f7c6