[Bundles] Fix gap during header swipe to expand

Fixes: 439537914

Test: swipe down from bundle header
      => single notif renders fine
      => group notif renders fine

      swipe down on group header inside bundle
      => 2nd level children render fine (no regression)

Test: NotificationChildrenContainerTest

Flag: com.android.systemui.notification_bundle_ui

Change-Id: I9eca78e51f811c600f00e3d999a0327063626638