Make DisplayContent the DisplayArea.Root

(4/n DisplayArea Sibling Roots)

Remove the unnecessary layers between DC and DA.Root.

Fix: 156425461
Test: manual: test magnification and screen rotation
Test: atest WmTests:DisplayContentTests
Test: atest CtsWindowManagerDeviceTestCases:MultiDisplaySystemDecorationTests
Change-Id: I9bd8b07e2c49c7d79ea31e943cb90b22fe5bdcb6