Implement WifiNetworkListFragment

1.Scan valid access points for DPP configurator by using WifiTracker.
2.Allow user to add a new network for DPP configurator.

Bug: 118794978
Test: RunSettingsRoboTests
Change-Id: I36504d28dec7d2c9091aa6c35ebe2495045681f7

res/xml/wifi_dpp_network_list.xml

@@ -0,0 +1,24 @@
+<?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.
+-->
+
+<PreferenceScreen
+    xmlns:android="http://schemas.android.com/apk/res/android">
+
+    <PreferenceCategory
+        android:key="access_points"
+        android:layout="@layout/preference_category_no_label"/>
+</PreferenceScreen>