Simplify out path of checked vendor property_contexts

Using Join with the fully fledged input path as string
breaks setting a custom $OUT_DIR

Test: export OUT_DIR=`pwd`/out_custom && m nothing
Change-Id: Ie5043c0eb8e5f854be0d0d318008ea24f3d94c09
diff --git a/build/soong/selinux_contexts.go b/build/soong/selinux_contexts.go
index e542c3a..5cc9c70 100644
--- a/build/soong/selinux_contexts.go
+++ b/build/soong/selinux_contexts.go
@@ -388,7 +388,7 @@
 		cmd.Flag("--strict")
 	}
 
-	out := pathForModuleOut(ctx, "namespace_checked").Join(ctx, input.String())
+	out := pathForModuleOut(ctx, ctx.ModuleName()+"_namespace_checked")
 	rule.Command().Text("cp -f").Input(input).Output(out)
 	rule.Build("check_namespace", "checking namespace of "+ctx.ModuleName())
 	return out