[flexiglass] Notification composables now set the particular placeholder...

[flexiglass] Notification composables now set the particular placeholder values from the right place.

* Only the ConstrainedNotificationStack sets the constrained height used for keyguard calculation
* The stack placeholder element sets the stack top and bottom
* The scrim bounds are now actually set by the scrim element

Bug: 296118689
Test: atest SystemUITests
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: Ic5ab3f387a6c15088dcc631e06f0909d422d5c98