Define a new BACKPORTED vehicle property group.

This vehicle property group is used for reserving some property IDs
that vendor can optionally implement as a custom property in older
Android released so that we can backport VHAL property features.

Test: m android.hardware.automotive.vehicle.property-update-api
Bug: 300700901
Change-Id: Ic1f4865466acfcabbecf8a70f3cbcfb6440c9fe6