Refactor and further simplify the RRO logic

Make the runtime vs static resource overlays a little clearer.
This will help adding more logic around determining if an RRO
needs to be generated in /vendor, /product or both.

Bug: 127758779
Test: verify noop on presubmit targets
Change-Id: I43111a1d9bb3405c559faaef56a75a5ad7672ba0