Add support for transforming a property set after its contents

This change adds support for transforming a property set after its
contents. This allows a transform to recursively prune empty property
sets that were created for a module.

The transformPropertySet method was renamed to
transformPropertySetBeforeContents and a new
transformPropertySetAfterContents method was added.

Bug: 148933848
Test: m nothing
Change-Id: Ia198d47e042b98c69406db4bc12859869816a387