Implement default assist app setting

- Add "None" support to AppListPreference
- Add DefaultAssistPreference to manage assist;
- Add AppListPreferenceWithSettings to show a settings icon;
- Implement DefaultAssistPreference based on AppListPreferenceWithSettings;
- Move voice input settings into ManageAssist and implement it
  based on AppListPreferenceWithSettings;

Bug:20210110
Change-Id: If283b8b55a46b428ecfa6e45dc2123292b1d4302

res/drawable/ic_remove_circle.xml

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2015 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.
+-->
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:height="48dp"
+    android:width="48dp"
+    android:tint="#db4437"
+    android:viewportHeight="48"
+    android:viewportWidth="48" >
+    <path android:fillColor="@android:color/white"
+       android:pathData="M24,4C12.95,4,4,12.95,4,24
+                         s8.95,20,20,20,20-8.95,20-20
+                         S35.05,4,24,4zm10,22H14v-4h20v4z"/>
+</vector>