Enforce visibility for internal libs.

Bug: 135686713
Test: try using these elsewhere
Change-Id: I64a85f744230536a0c362138ad1aa71f19f58c01
diff --git a/Android.bp b/Android.bp
index c0058ef..626f207 100644
--- a/Android.bp
+++ b/Android.bp
@@ -102,6 +102,8 @@
     whole_static_libs: [
         "libhwbinder_pgo-impl-internal",
     ],
+
+    visibility: ["//system/libhwbinder:__subpackages__"],
 }
 
 // WARNING: deprecated