Add default_visibility for soong modules

So that we don't get surprise usages.

Bug: 348717861
Test: m nothing
Change-Id: I47319a727092c1bd936cca88f5713ee977a48b48

apex/Android.bp

@@ -42,4 +42,6 @@ bootstrap_go_package {
         "systemserver_classpath_fragment_test.go",
     ],
     pluginFor: ["soong_build"],
+    // Used by plugins
+    visibility: ["//visibility:public"],
 }