ambient: check enabled when dismissing UI

Bug: b:168817491
Change-Id: I4269b354c686613cc172b2e5ffdac6acd27ab780
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464008
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815842}

ash/ambient/ambient_controller.cc

@@ -429,6 +429,11 @@ void AmbientController::ShowUi() {
 void AmbientController::ShowHiddenUi() {
   DVLOG(1) << __func__;
 
+  if (!IsAmbientModeEnabled()) {
+    LOG(WARNING) << "Ambient mode is not allowed.";
+    return;
+  }
+
   ambient_ui_model_.SetUiVisibility(AmbientUiVisibility::kHidden);
 }
 
@@ -510,10 +515,17 @@ void AmbientController::RequestAccessToken(
 }
 
 void AmbientController::DismissUI() {
-  if (LockScreen::HasInstance())
-    ShowHiddenUi();
-  else
+  if (!IsAmbientModeEnabled()) {
     CloseUi();
+    return;
+  }
+
+  if (LockScreen::HasInstance()) {
+    ShowHiddenUi();
+    return;
+  }
+
+  CloseUi();
 }
 
 AmbientBackendModel* AmbientController::GetAmbientBackendModel() {