WM: Introduce DisplayArea (3/n)

Introduces the concept of a DisplayArea, corresponding to an area
on a logical display within which content can be placed.

DisplayAreas can contain more DisplayAreas, WindowTokens and ActivityStacks.

A future CL will expose an API which allows leashing a DisplayArea to transform
all its content simultaneously.

DisplayAreas are managed by a DisplayAreaPolicy, which creates and places the
areas, and decides which area a container is placed in.

This CL introduces the concept, but applies a no-op policy that maintains the
current orderings and does not apply any overrides.

Future work that remains:
- Writing the feature policies
- Adjusting Display & DisplayMetrics if the area is constraining its children.
- Moving the policy into an product-adjustable component

Bug: 147406652
Test: atest WmTests
Change-Id: If6fb1bba3b65ebf7ac9fdf99408c54bf77f602c9