res/anim/fade_out.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2014 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.
-->

<!-- startOffset is the same as the duration of the wallpaper_enter animation. We have this delay so
    that we don't see the wallpaper changing before fading back to the home screen. -->
<alpha xmlns:android="http://schemas.android.com/apk/res/android"
    android:startOffset="@android:integer/config_longAnimTime"
    android:duration="@android:integer/config_mediumAnimTime"
    android:fromAlpha="1"
    android:toAlpha="0"/>