Traverse window hierarchy without window list

Added support for to get all windows in the hierarchy without needing
to use WindowList concept which is a very complicated implementation
in the code base.
This implementation walks the hierarchy node by node returns windows
in order to the caller using a callback.

Test: bit FrameworksServicesTests:com.android.server.wm.DisplayContentTests
Change-Id: I2719f7c96f26dad23f91c1c589be88712bd224b8