Set per-partition timestamps in OTA generation

update_engine can also accept payload with per-partition timestamps.
This CL updates OTA generation script to emit per-partition timestamps
when writing an OTA package.

Test: Generate && serve an ota
Change-Id: I17529a004b8e0bbcb7d69dde93fb0fd7124b3b17
diff --git a/payload_generator/payload_file.h b/payload_generator/payload_file.h
index d1f8196..3dce00f 100644
--- a/payload_generator/payload_file.h
+++ b/payload_generator/payload_file.h
@@ -96,6 +96,8 @@
 
     PostInstallConfig postinstall;
     VerityConfig verity;
+    // Per partition timestamp.
+    std::string version;
   };
 
   std::vector<Partition> part_vec_;