AAPT2: Enable building proto artifacts

- This will allow the bundle tool to ingest the outputs of the aapt2 link
  phase.

Bug: 64143208
Test: manual
Change-Id: I1a4b3ce5c2ffbbdc4bc642c3371a9ef2e7e9ad71
diff --git a/tools/aapt2/ResourceUtils.cpp b/tools/aapt2/ResourceUtils.cpp
index 6fac6e9..24187d9 100644
--- a/tools/aapt2/ResourceUtils.cpp
+++ b/tools/aapt2/ResourceUtils.cpp
@@ -704,8 +704,15 @@
       } else {
         if (type != ResourceType::kString && util::StartsWith(str, "res/")) {
           // This must be a FileReference.
-          return util::make_unique<FileReference>(dst_pool->MakeRef(
-              str, StringPool::Context(StringPool::Context::kHighPriority, config)));
+          std::unique_ptr<FileReference> file_ref =
+              util::make_unique<FileReference>(dst_pool->MakeRef(
+                  str, StringPool::Context(StringPool::Context::kHighPriority, config)));
+          if (util::EndsWith(*file_ref->path, ".xml")) {
+            file_ref->type = ResourceFile::Type::kBinaryXml;
+          } else if (util::EndsWith(*file_ref->path, ".png")) {
+            file_ref->type = ResourceFile::Type::kPng;
+          }
+          return std::move(file_ref);
         }
 
         // There are no styles associated with this string, so treat it as a simple string.