assistant: Do not show Web UI for embedded mode

|kDeepLink| will open embedded UI but we have not defined the UX to show
the Web UI. Current guard will not prevent this happens. This patch
modify the guard to eary return when the embedded assistant UI is enabled.

Bug: 924624
Test: manual
Change-Id: Idb44231cd66b07d85c6c400d4b2c152813b227ce
Reviewed-on: https://chromium-review.googlesource.com/c/1487515Reviewed-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@{#636236}

ash/assistant/assistant_ui_controller.cc

@@ -236,7 +236,7 @@ void AssistantUiController::OnDeepLinkReceived(
     return;
 
   // TODO(wutao): Behavior is not defined.
-  if (model_.ui_mode() == AssistantUiMode::kLauncherEmbeddedUi)
+  if (app_list_features::IsEmbeddedAssistantUIEnabled())
     return;
 
   ShowUi(AssistantEntryPoint::kDeepLink);