Only generate kythe xrefs on the primary module

Currently the kythe build is hitting the 6 hour timeout, but we're
generating kythe files for all variants. Generating them only for
the primary variant should drastically cut its runtime.

Bug: 345440993
Test: Presubmits
Change-Id: I3846b25c7f0bc22153e4f0a0cb81c5015854dd70

cc/builder.go

@@ -485,7 +485,7 @@ func transformSourceToObj(ctx ModuleContext, subdir string, srcFiles, noTidySrcs
 		coverageFiles = make(android.Paths, 0, len(srcFiles))
 	}
 	var kytheFiles android.Paths
-	if flags.emitXrefs {
+	if flags.emitXrefs && ctx.Module() == ctx.PrimaryModule() {
 		kytheFiles = make(android.Paths, 0, len(srcFiles))
 	}
 
@@ -664,7 +664,7 @@ func transformSourceToObj(ctx ModuleContext, subdir string, srcFiles, noTidySrcs
 		})
 
 		// Register post-process build statements (such as for tidy or kythe).
-		if emitXref {
+		if emitXref && ctx.Module() == ctx.PrimaryModule() {
 			kytheFile := android.ObjPathWithExt(ctx, subdir, srcFile, "kzip")
 			ctx.Build(pctx, android.BuildParams{
 				Rule:        kytheExtract,