[Flexiglass] Shrink the Constrained Notification Placeholder to the stack height

The magic portrait feature needs to know the bottom-most position of
Notifications on the Lockscreen to know, where can it place its
highlighted image. By shrinking the placeholder to the stack size, we
are able to use its bottom bounds to determine the available space,
instead of recalculating the stack height again.

Bug: 391405643
Flag: com.android.systemui.scene_container
Test: check Notification positioning on the Lockscreen

Change-Id: Ied59bd70e713047b723fb30995fcf932a4155dad