[flexiglass] Fix NSSL scrolling in split shade and lockscreen to splitshade transition.

Also turns off NSSL ignoring width of notifications when expanding them, greatly improving landscape shade usability. This behavior diverges from legacy but is a clear improvement.

Bug: 338080993
Bug: 335828150
Test: visually verified lockscreen -> split shade transition
Test: visually verified NSSL scrolling with overflowing notifications
Test: added unit test
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: I43b6c0bb5db6fa820a720e6e1d2f6444b972d5f8