Use result.Config.PrebuiltOS() to get prebuiltHost in test_spec_test

Bug: 312536783
Bug: 312536905
Test: m nothing --no-skip-soong-tests -j96

Ignore-AOSP-First: CPing test_spec rule to udc-mainline-prod to support migration of test targets. Cherry pick of:aosp/2836072

Change-Id: I1e12281927269d42ae796348b223030acfd6ecfa
Merged-In: I1e12281927269d42ae796348b223030acfd6ecfa