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: I9329cae35cc79ee35ea0e33929bef15d36b08ca5
diff --git a/Android.bp b/Android.bp
index 6535c2c..6523ac9 100644
--- a/Android.bp
+++ b/Android.bp
@@ -24,6 +24,10 @@
 
 java_library {
     name: "apache-xml",
+    visibility: [
+        "//art/build/apex",
+        "//libcore",
+    ],
     srcs: ["src/main/java/**/*.java"],
     java_resource_dirs: ["src/main/java"],