Also set system version to ForcedUpdate.

`update_engine_client --update` will set the app version to
ForcedUpdate, but the system version should be overriden too,
if exists.

Bug: 73896093
Test: update_engine_client --update
Change-Id: Ibd41eecacb5312d6f77d5e0c33af090aca38e054
diff --git a/omaha_request_params.cc b/omaha_request_params.cc
index 9e78a93..97a2ed1 100644
--- a/omaha_request_params.cc
+++ b/omaha_request_params.cc
@@ -77,10 +77,14 @@
   LOG(INFO) << "Running from channel " << image_props_.current_channel;
 
   os_platform_ = constants::kOmahaPlatformName;
-  if (!image_props_.system_version.empty())
+  if (!image_props_.system_version.empty()) {
+    if (in_app_version == "ForcedUpdate") {
+      image_props_.system_version = in_app_version;
+    }
     os_version_ = image_props_.system_version;
-  else
+  } else {
     os_version_ = OmahaRequestParams::kOsVersion;
+  }
   if (!in_app_version.empty())
     image_props_.version = in_app_version;