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/ResourceValues.h b/tools/aapt2/ResourceValues.h
index 7e7547f..275864b 100644
--- a/tools/aapt2/ResourceValues.h
+++ b/tools/aapt2/ResourceValues.h
@@ -292,7 +292,7 @@
 };
 
 struct Array : public BaseValue<Array> {
-  std::vector<std::unique_ptr<Item>> items;
+  std::vector<std::unique_ptr<Item>> elements;
 
   bool Equals(const Value* value) const override;
   Array* Clone(StringPool* new_pool) const override;