include build timestamp in OTA metadata

Change-Id: I44292c6346db3356dfafbd044027e37a9b467cfe
diff --git a/tools/releasetools/ota_from_target_files b/tools/releasetools/ota_from_target_files
index cf5fb34..932d5b0 100755
--- a/tools/releasetools/ota_from_target_files
+++ b/tools/releasetools/ota_from_target_files
@@ -355,6 +355,7 @@
 
   metadata = {"post-build": GetBuildProp("ro.build.fingerprint", input_zip),
               "pre-device": GetBuildProp("ro.product.device", input_zip),
+              "post-timestamp": GetBuildProp("ro.build.date.utc", input_zip),
               }
 
   device_specific = common.DeviceSpecificParams(
@@ -602,6 +603,7 @@
     raise ValueError('unknown script mode "%s"' % (OPTIONS.script_mode,))
 
   metadata = {"pre-device": GetBuildProp("ro.product.device", source_zip),
+              "post-timestamp": GetBuildProp("ro.build.date.utc", target_zip),
               }
 
   device_specific = common.DeviceSpecificParams(