Assign display field when creating WindowState

This avoids a NPE trap in WindowManagerService#addWindow that
the WindowState is created but WindowToken#addWindow(WindowState)
is not call yet (setParent will update mDisplayContent).

Also make WindowContainer#getDisplayContent as a final method
because the hierarchy should sync the field.

Bug: 163976519
Test: CtsWindowManagerDeviceWindow

Change-Id: I5caea8288fcf0e960b9e1b7b36a847d8262d2fb1