Introduced ReadOnlyWindowList

7th and Final step in making the modification of a display's
WindowList private to DisplayContent.
ReadOnlyWindowList provides an interface for external classes to
DisplayContent to access the window list without being able to
modify it. This will be important in upcoming CLs where it is
important for us to keep track of when the window list changes.

Bug: 30060889
Test: Manual testing and existing tests pass.

Change-Id: I4de0b258a40fd4b21ef9cc9e3401488f76d25f83