Send both system id and product id to Omaha.

The request will have two <app> with different app_id and version.

Test: observe request in the log
Change-Id: I51b0dcf3c0affc81fe152ff24ce0ccc9a36e6385
(cherry picked from commit 573919d54a8421f1b2147d801ae4104adebf5cd7)
diff --git a/image_properties.h b/image_properties.h
index ba6ce44..ac6d474 100644
--- a/image_properties.h
+++ b/image_properties.h
@@ -33,9 +33,13 @@
   std::string product_id;
   // The canary-channel product id.
   std::string canary_product_id;
+  // The system id for the Android Things SoM, empty for Chrome OS.
+  std::string system_id;
 
   // The product version of this image.
   std::string version;
+  // The system version of this image.
+  std::string system_version;
 
   // A unique string that identifies this build. Normally a combination of the
   // the version, signing keys and build target.