AAPT2: Define intermediate compiled XML proto

This proto format is meant to encapsulate more information
that is specific to Android and allows for easier validation
and manipulation across tools.

Test: make aapt2_tests
Change-Id: I13bc34a460671fc0a36246be0d287a3d37d244d6
diff --git a/tools/aapt2/ValueVisitor.h b/tools/aapt2/ValueVisitor.h
index 2763d49..eb4fa49 100644
--- a/tools/aapt2/ValueVisitor.h
+++ b/tools/aapt2/ValueVisitor.h
@@ -80,7 +80,7 @@
   }
 
   void VisitSubValues(Array* array) {
-    for (std::unique_ptr<Item>& item : array->items) {
+    for (std::unique_ptr<Item>& item : array->elements) {
       item->Accept(this);
     }
   }