Don't pin notifications when the shade is expanded

Follow the behaviour in the legacy path of this refactor, and keep the
expanded state updated un the HeadsUpManager.

Fixes: 352042267
Test: HUN appears above the overscrolled stack
      Scroll the stack below the HUN
      => HUN animates to expanded as it gets more space with the scrolling

Flag: com.android.systemui.notifications_heads_up_refactor
Change-Id: I35e357535bb3ce86b65425327d269cf7d57f1f5a