Ignore Android.mk files for NDK build in external/

Bug: 318428689
Test: CIs
Change-Id: I3e9daf6a79872794db02e6278f32a3324c743bdd
diff --git a/ui/build/finder.go b/ui/build/finder.go
index 573df21..a899822 100644
--- a/ui/build/finder.go
+++ b/ui/build/finder.go
@@ -128,6 +128,7 @@
 
 	// Stop searching a subdirectory recursively after finding an Android.mk.
 	androidMks := f.FindFirstNamedAt(".", "Android.mk")
+	androidMks = ignoreNdkAndroidMks(androidMks)
 	blockAndroidMks(ctx, androidMks)
 	err := dumpListToFile(ctx, config, androidMks, filepath.Join(dumpDir, "Android.mk.list"))
 	if err != nil {