update_engine: remove system_id and system_version

These two variables and their use cases was introduced for android, but
eventually android abandon it for some other solution. So this are stale
now.

Also removed the implemnation of ImageProperties for Android as it
doesn't seem Android is actually using it.

BUG=b:171829801
TEST=unittests

Change-Id: Ic793cfa5031d69b5390acefb1b8cd75291708890
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2505885
Reviewed-by: Jae Hoon Kim <[email protected]>
Reviewed-by: Tianjie Xu <[email protected]>
Reviewed-by: Amin Hassani <[email protected]>
Tested-by: Amin Hassani <[email protected]>
Commit-Queue: Amin Hassani <[email protected]>

Author: gigilibala2

image_properties.h

@@ -33,13 +33,9 @@ struct ImageProperties {
   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;
 
   // The version of all product components in key values pairs.
   std::string product_components;

image_properties_android.cc

@@ -865,11 +865,6 @@ bool OmahaRequestAction::ParseParams(OmahaParserData* parser_data,
     if (app.id == params_->GetAppId()) {
       // this is the app (potentially the only app)
       output_object->version = app.manifest_version;
-    } else if (!params_->system_app_id().empty() &&
-               app.id == params_->system_app_id()) {
-      // this is the system app (this check is intentionally skipped if there is
-      // no system_app_id set)
-      output_object->system_version = app.manifest_version;
     } else if (params_->is_install() &&
                app.manifest_version != params_->app_version()) {
       LOG(WARNING) << "An app has a different version (" << app.manifest_version