recovery: allow A/B updater to downgrade

Change-Id: Iaa1fb7838fb958e69fb3104fef7743aafad12b1b
diff --git a/recovery.cpp b/recovery.cpp
index fe2ea3e..8635441 100644
--- a/recovery.cpp
+++ b/recovery.cpp
@@ -178,6 +178,15 @@
   }
 }
 
+bool ask_to_continue_downgrade(Device* device) {
+  if (get_build_type() == "user") {
+    return false;
+  } else {
+    device->GetUI()->SetProgressType(RecoveryUI::EMPTY);
+    return yes_no(device, "This package will downgrade your system", "Install anyway?");
+  }
+}
+
 static bool ask_to_wipe_data(Device* device) {
   std::vector<std::string> headers{ "Format user data?", "This includes internal storage.", "THIS CANNOT BE UNDONE!" };
   std::vector<std::string> items{ " Cancel", " Format data" };