[Flexiglass] Convert NotificationStackCutoffGuideline to a Rect

Shrink the NotificationPlaceHolder's height by its y offset in SingleShade,
so that it can only grow to the visible space, where the stack needs to be shown.

Use the NotificationPlaceHolder's bounds to communicate a drawing area
to the NSSL.

Retire the NotificationStackCutoffGuideline, and use the new bounds to
determine the bottom-most position of the stack (aka the shelf's place).

Bug: 417965077
Test: check the box drawn by the debug lines in all shade modes
Flag: com.android.systemui.scene_container
Change-Id: I09489ea3f0f52ac3219920a4a42697ae4d231736