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/apex/Android.bp b/apex/Android.bp
index ef2f755..4848513 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -42,4 +42,6 @@
         "systemserver_classpath_fragment_test.go",
     ],
     pluginFor: ["soong_build"],
+    // Used by plugins
+    visibility: ["//visibility:public"],
 }