libbinder_ndk: test vendor/system stability

A vendor stability binder shouldn't be able to call into a system
stability binder. Before, we were missing a negative test.

Bug: 136027762
Test: binderVendorDoubleLoadTest
Change-Id: I4944c5c61d42873798dac9c132b743469a80dd63
diff --git a/libs/binder/Android.bp b/libs/binder/Android.bp
index 643a956..2a140f4 100644
--- a/libs/binder/Android.bp
+++ b/libs/binder/Android.bp
@@ -164,3 +164,16 @@
     ],
     path: "aidl",
 }
+
+aidl_interface {
+    name: "libbinder_aidl_test_stub",
+    local_include_dir: "aidl",
+    srcs: [":libbinder_aidl"],
+    visibility: [":__subpackages__"],
+    vendor_available: true,
+    backend: {
+        java: {
+            enabled: false,
+        },
+    },
+}