[Flexiglass] Let the ConstrainedNotificationStack fillMaxSize()

Let the ConstrainedNotificationStack grow full size, so it can signal
the available space for the maxDisplayedNotifications calculation.
Let the NotificationPlaceholder continue to be wrapped to its content
height.

Fixes: 424796260
Bug: 391405643
Flag: com.android.systemui.scene_container
Test: receive new notifications in the LS -> check if they can fill in the available space

Change-Id: I55bbf31bd2dc6c3b183c89efaf654c6071846a25