Introduce DisplayAreaHierarchyBuilder

(5/n DisplayArea Sibling Roots)

Refactor DisplayAreaPolicyBuilder to use it for creating display area
hierarchy below root.

The feature and hierarchy info will be stored in RootDisplayArea.

Bug: 157683117
Test: manual: test rotation and magnification
Test: atest WmTests:DisplayAreaPolicyBuilderTest
Test: atest WmTests:DisplayAreaProviderTest
Test: atest WmTests:WindowManagerServiceTests
Change-Id: I5b1f6dab2eb4061501c11548cac1764753378b3e