[flexiglass] Break up lockscreen sections into elements

The concept of "section" comes from the non-Compose version of the
lockscreen that used ConstraintLayout and is no longer relevant or
useful in the world of a Compose-based custom layout where each element
is measured, placed, and drawn by a centralized slot-based
LockscreenSceneLayout.

Bug: 395060439
Bug: 405383415
Bug: 396218588
Bug: 396202921
Bug: 384077475
Bug: 394875986
Bug: 405366494
Bug: 390988286
Test: manually verified no visible changes
Flag: com.android.systemui.scene_container
Change-Id: Ia7c7537767fec66bbbd97fe531ed6425a1b4541d