[Flexiglass] Fix Notification removal animations

This CL fixes a timing issue, between the NSSL updating its content
height, and the NotificationPlaceholder running its layout to follow up
on the new height value. It is achieved by:
 - replacing the height modifier on the placeholder with a layout one
 - reading the intrinsic content height directly from the NSSL
 - making the NSSL to invalidate the layout of the placeholder, whenever
   its height is changing
 - creating a LayoutModifierNode instead of a State, to make this
   happen within one frame

Fixes: 330667757
Test: Have lots of Notifications. Scroll to the bottom of the shade, dismiss a Notification. Observe
the animation.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I66ad42907d2725f5fb4a169f60dc94a6e87ce56b