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.cc b/payload_generator/payload_file.cc
index c1594c7..1388f2d 100644
--- a/payload_generator/payload_file.cc
+++ b/payload_generator/payload_file.cc
@@ -92,6 +92,7 @@
   part.aops = std::move(aops);
   part.postinstall = new_conf.postinstall;
   part.verity = new_conf.verity;
+  part.version = new_conf.version;
   // Initialize the PartitionInfo objects if present.
   if (!old_conf.path.empty())
     TEST_AND_RETURN_FALSE(
@@ -132,6 +133,9 @@
   for (const auto& part : part_vec_) {
     PartitionUpdate* partition = manifest_.add_partitions();
     partition->set_partition_name(part.name);
+    if (!part.version.empty()) {
+      partition->set_version(part.version);
+    }
     if (part.postinstall.run) {
       partition->set_run_postinstall(true);
       if (!part.postinstall.path.empty())