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
Bug: 65645766
Change-Id: I13bc34a460671fc0a36246be0d287a3d37d244d6
Merged-In: I13bc34a460671fc0a36246be0d287a3d37d244d6
diff --git a/tools/aapt2/ResourceParser.cpp b/tools/aapt2/ResourceParser.cpp
index a5783a5..1c3ac2a 100644
--- a/tools/aapt2/ResourceParser.cpp
+++ b/tools/aapt2/ResourceParser.cpp
@@ -1219,7 +1219,7 @@
         continue;
       }
       item->SetSource(item_source);
-      array->items.emplace_back(std::move(item));
+      array->elements.emplace_back(std::move(item));
 
     } else if (!ShouldIgnoreElement(element_namespace, element_name)) {
       diag_->Error(DiagMessage(source_.WithLine(parser->line_number()))