Since the genrule tool dep mutator runs after the prebuilt mutators, this adds a helper function android.PrebuiltGetPreferred to resolve the source or prebuilt as appropriate. Test: m SOONG_CONFIG_art_module_source_build=false droid in internal Bug: 214292395 Change-Id: I1a208fd048b998f9f19ad1f45d8389decda2cb9e