app_list: Reset model state on closing

When Launcher is closed when embedded Assistant Ui visible, need to
reset the model AppListState to make it consistent with the
assistant_ui_controller state.

Bug: 970039
Test: manual
Change-Id: Iab119f3267d62a5eb3a4f2456e1c14c7e38ba0d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642963Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666045}

ash/app_list/app_list_controller_impl.cc

@@ -1007,6 +1007,7 @@ void AppListControllerImpl::ViewClosing() {
   }
 
   CloseAssistantUi(AssistantExitPoint::kLauncherClose);
+  model_->SetState(AppListState::kInvalidState);
   if (client_)
     client_->ViewClosing();
 }