Tweak splitshade nssl positioning

Specifically for keyguard/shade migration, the top padding in split
shade mode needed to match the legacy positioning in order to prevent
the shade jumping slightly when pulled down on lockscreen. This also
ensures the placeholder is properly sized so the max notification
logic works.

Bug: 308772985
Test: atest SharedNotificationContainerInteractorTest
SharedNotificationContainerViewModelTest
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
DEVELOPMENT

Change-Id: I11cc76f207d98d7d817f470001690b4d96566ed6