[flexiglass] Support for split layout bouncer.

We want to support a case where the layout of the bouncer is split such
that the message and emergency button appear on one side while the input
entry area appears on the other side. This CL achieves that.

Bug: 304630105
Test: See screenshots in bug
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: Ib9c3a8e1bc030068eb386e50c39d230e2e13285b