Always powerwash on channel change if arbitrary channel allowed.

Merged is_powerwash_allowed() and to_more_stable_channel() into
ShouldPowerwash().

Bug: 73082835
Test: update_engine_unittests
Change-Id: I6b7af0d1dac28d5fa9cddf4391bbd9cdf2acb57b
diff --git a/omaha_response_handler_action.cc b/omaha_response_handler_action.cc
index 9c5fb4a..2d6105a 100644
--- a/omaha_response_handler_action.cc
+++ b/omaha_response_handler_action.cc
@@ -139,7 +139,7 @@
   system_state_->prefs()->SetString(current_channel_key,
                                     params->download_channel());
 
-  if (params->to_more_stable_channel() && params->is_powerwash_allowed())
+  if (params->ShouldPowerwash())
     install_plan_.powerwash_required = true;
 
   TEST_AND_RETURN(HasOutputPipe());