libbinder: limit visibility of test ifaces

More explicit since people were asking to use them. The four AIDL
backends have native language APIs for service management.

Bug: N/A
Test: N/A
Change-Id: Id5dd9af8bbbcc6c56fa0ab5ddbb6cfa517566aca
diff --git a/libs/binder/Android.bp b/libs/binder/Android.bp
index eb130de..eb019d4 100644
--- a/libs/binder/Android.bp
+++ b/libs/binder/Android.bp
@@ -522,6 +522,10 @@
             enabled: false,
         },
     },
+    visibility: [
+        ":__subpackages__",
+        "//system/tools/aidl:__subpackages__",
+    ],
 }
 
 // TODO(b/184872979): remove once the Rust API is created.