[flexiglass] SmartSpaceSection in Compose.

Implements the smartspace section for the new blueprint/section system
used by Flexiglass' Lockscreen scene.

Note: the burn-in solution is still missing, that will come in a
followup CL.

Bug: 316211368
Test: manually verified that the date, weather, and next calendar event
card are all showing properly in the lockscreen scene of Flexiglass
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I080762c5f345914aa6c6c5d329c02a274042dc7e