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)