[roll] Roll fuchsia [flatland] Step 070: UberStruct schema and safety flag

Adds UberStructLayer to the UberStruct schema along with new allocation
maps (`layer_stacks` and `layers`). This data is unused by any readers
but can be successfully round-tripped and snapped.

Also introduces the global `use_flatland2_uberstruct_schema` flag to
the FlatlandConfig and scenic-impl.shard.cml. FlatlandManager uses
this flag to strictly assert structural exclusivity between the classic
Flatland1 array data and the new Flatland2 maps in UberStructs.

Testing:
- Added LayerStacksSurviveSnapshot and UberStructLayerFieldsRoundTrip to
  uber_struct_system_unittest.
- Noted: Cannot test FlatlandManager overriding Flatland configs directly,
  as the config is not observable over FIDL. Will indirectly assert this
  safety behavior in Step 080.

Original-Original-Bug: 523371761
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1661290
Original-Original-Revision: 5da1d710938f0e8eed7b3a917011823e7f694685
GitOrigin-RevId: cd217adc1603bdcc31558ba33cfb12729b5db14d
Change-Id: Id536a700a97a9393e7ab1244f6c2dd236ab7d6cc
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1697256
Cr-Commit-Position: refs/heads/main@{#196356}