Convert int to string for payload sizes

.. to handle larger values without overflowing.

Bug: 312749584
Test: m + atest

Change-Id: I25b46167d657835c565661837649841fafdd3a4f
diff --git a/payload_generator/payload_properties.h b/payload_generator/payload_properties.h
index 846b181..9aeb5f8 100644
--- a/payload_generator/payload_properties.h
+++ b/payload_generator/payload_properties.h
@@ -52,11 +52,11 @@
   // format, this needs to be increased.
   int version_{2};
 
-  size_t metadata_size_;
+  uint64_t metadata_size_;
   std::string metadata_hash_;
   std::string metadata_signatures_;
 
-  size_t payload_size_;
+  uint64_t payload_size_;
   std::string payload_hash_;
 
   // Whether the payload is a delta (true) or full (false).