DND should be displayed on search as slice.

Add ZenModeSoundSettingsPreferenceController extends ZenModePreferenceController,
and update availability status to AVAILABLE.

This is cherry-picked from ag/6897872

Change-Id: I8dcb6808cc21c4f5c06548a363c998e369646487
Bug: 129320642
Test: manual & robotests

src/com/android/settings/notification/ZenModeSoundSettingsPreferenceController.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License
+ */
+
+package com.android.settings.notification;
+
+import android.content.Context;
+
+public class ZenModeSoundSettingsPreferenceController extends ZenModePreferenceController {
+
+    public static final String ZEN_MODE_KEY = "zen_mode";
+
+    public ZenModeSoundSettingsPreferenceController(Context context, String key) {
+        super(context, key);
+    }
+
+    @Override
+    public int getAvailabilityStatus() {
+        return AVAILABLE;
+    }
+}