Restrict visibility of framework-module-common-defaults

This isn't meant to be used directly, instead one of the default
modules that inherits this rule should be used.

Test: m nothing
Change-Id: I32a20de74c29b9fa1a0083bce084896aad99eb41

apex/Android.bp

@@ -112,6 +112,8 @@ java_defaults {
     ],
     stubs_source_visibility: ["//visibility:private"],
 
+    defaults_visibility: ["//visibility:private"],
+
     // Collates API usages from each module for further analysis.
     plugins: ["java_api_finder"],