AAPT2: Move all file output to FileOutputStream

FileOutputStream is safe to use on Windows, as it opens
files using our compatibility API.

Bug: 68262818
Test: make aapt2_tests
Change-Id: Ib0b27e93edd609b49b1327db7d9867a002198ebb
diff --git a/tools/aapt2/Resource.h b/tools/aapt2/Resource.h
index 96a0203..6fcf0f6 100644
--- a/tools/aapt2/Resource.h
+++ b/tools/aapt2/Resource.h
@@ -268,6 +268,11 @@
   return out << res_id.to_string();
 }
 
+// For generic code to call 'using std::to_string; to_string(T);'.
+inline std::string to_string(const ResourceId& id) {
+  return id.to_string();
+}
+
 //
 // ResourceType implementation.
 //