[flexiglass] Notification section in pure Compose.

Reuses existing work that uses a composable for the NSSL to redo how we
communicate the placement of the NSSL to the notification system.

This is using the same solution as what other Flexiglass scenes are
using.

Bug: 316211368
Test: turned on the legacy flag NSSL_DEBUG_LINES that renders a red
background and verified that it's properly placed in the hierarchy
between the smart space and the lock icon.
Test: posted notifications using the notifier app and saw them if I also
turned on FLEXI_NOTIFS in SceneContainerFlagsExtension.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: Icab4b9cf4ff3a6a5cc249444b803c51ab99de252