Introduce new gap height between buckets and fix bundle gaps

In Figma the gaps between buckets are 4dp but so far we had no
mechanism that creates such gaps. This introduces gaps between buckets
to be 4dp and fixes the gaps between bundles being too large (16dp).

Bug: b/416474210
Test: Verified in hsv that gaps between buckets (and bundles) are now
      4dp
Flag: com.android.systemui.notification_bundle_ui
Change-Id: I0d8b443cf9083f8b70ced152b196aa9a624c8dc1