Add core library modules visibility rules

Adds visibility to core library modules to prevent them from being
accessed unnecessarily.

Bug: 112158820
Test: make core-tests
Change-Id: I8dae23b70fb1bbbbc60b0d010526afeacdb8fc74
diff --git a/Android.bp b/Android.bp
index e9f1dde..f60c50d 100644
--- a/Android.bp
+++ b/Android.bp
@@ -48,6 +48,11 @@
 
 java_library {
     name: "okhttp",
+    visibility: [
+        "//art/build/apex",
+        "//external/robolectric-shadows",
+        "//libcore",
+    ],
     srcs: [
         // Although some of the classes in the android/ directory are already in the correct
         // package and do not need to be moved to another package they are transformed as they