Add a platform compat config schema API.

As decribed in go/xsdc-for-partners.
This defines the schema of the XML file generated by @ChangeId
annotation processor.
The schema requires unique ids.

Test: Used https://www.freeformatter.com/xml-validator-xsd.html to
      validate an example config.
Bug: 138222363

Change-Id: Iaf37e049ddd483c4fd7d512475614476ac6606a5