Introduce DisplayAreaGroup

(7/n DisplayArea Sibling Roots)

Allow OEM to create DisplayAreaGroup when building DisplayAreaPolicy.

Bug: 157683117
Fix: 159144430
Test: atest WmTests:WindowTokenTests
Test: atest WmTests:DisplayAreaPolicyBuilderTest
Change-Id: I3cf5c10398f4ea4dec224633e17483fe7d063cf6
9 files changed