Refactor and improve window layering.

This CL moves layer calculation into a separate class, so the whole
logic can be encapsulated. It also extracts special cases from the
general loop and instead applies them at the end. This simplifies the
logic in the main algorithm and makes it clearer what needs to happen
for special cases.

Bug: 26144888

Change-Id: I87347bf0198bd0d3cd09e4231b4652ab979f2456