Extract new system version from Omaha responses

When processing an Omaha response, line up the app elements for the main
product and system apps, by their appid fields.  This allows the (product)
version and system version to be differentiated and recorded in the
InstallPlan.

Bug: 65422956
Test:  Unit-tests, and using a live OTA
Change-Id: I5217b7ac6fbf6f7ae595c1cec6fbc329051925eb
(cherry picked from commit 19ef6db4885387a5e3fbd2affcbd57a9a40d8b9b)
diff --git a/payload_consumer/install_plan.h b/payload_consumer/install_plan.h
index 6dd5a73..551f8c2 100644
--- a/payload_consumer/install_plan.h
+++ b/payload_consumer/install_plan.h
@@ -54,6 +54,8 @@
   bool is_resume{false};
   std::string download_url;  // url to download from
   std::string version;       // version we are installing.
+  // system version, if present and separate from version
+  std::string system_version;
 
   struct Payload {
     uint64_t size = 0;               // size of the payload