Add default_visibility for soong modules

So that we don't get surprise usages.

Bug: 348717861
Test: m nothing
Change-Id: I47319a727092c1bd936cca88f5713ee977a48b48
diff --git a/java/config/Android.bp b/java/config/Android.bp
index bfe83ab..6217390 100644
--- a/java/config/Android.bp
+++ b/java/config/Android.bp
@@ -17,4 +17,8 @@
         "kotlin.go",
         "makevars.go",
     ],
+    visibility: [
+        "//build/soong:__subpackages__",
+        "//external/error_prone/soong",
+    ],
 }