Let MasterSwitchPreference be restrictable

Let MasterSwitchPreference be restrictable by extending RestrictedPreference.
This is a prerequisite CL for "Hotspot & Tethering" in "Network & Internet" settings screen to be a MasterSwtichPreference.
Also changed MasterSwitchPreferenceTest perse.

Bug: 145922022
Test: SettingsRoboTests except StorageItemPreferenceControllerTest which
fails before this CL.
Change-Id: I54fa59d7adfce5f564c178c0db334974dbabf059

res/layout/restricted_preference_widget_master_switch.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  ~ 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.
+  -->
+
+<merge xmlns:android="http://schemas.android.com/apk/res/android">
+    <include layout="@layout/restricted_icon"/>
+
+    <include layout="@layout/preference_widget_master_switch"/>
+</merge>