assistant: fix crash when trying to hide non-exist Assistant

When assistant controllers init and receives an assistant status of
NOT_READY, it tries to hide non-exist Assistant, which will eventually
cause a segfault.

Bug: 901720
Test: locally build and test
Change-Id: I4bf7247c175906167436849b4514c450c763c79f
Reviewed-on: https://chromium-review.googlesource.com/c/1318680Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605524}

ash/assistant/assistant_ui_controller.cc

@@ -318,7 +318,7 @@ void AssistantUiController::ShowUi(AssistantSource source) {
 }
 
 void AssistantUiController::HideUi(AssistantSource source) {
-  if (model_.visibility() == AssistantVisibility::kHidden)
+  if (model_.visibility() != AssistantVisibility::kVisible)
     return;
 
   if (container_view_)