Android P添加屏保功能，如果休眠时间设定大于屏保时间，则先进入屏保，达到休眠时间后再进入休眠

需求：
1.添加屏幕互保开关，默认关闭；
2.只保留时钟，可设定指针和数字、夜间模式；
3.启用时间改多长时间无操作进入屏保，可设定1、5、10、15、30分钟；
4.如果休眠时间设定小于屏保时间，则优先进入休眠，如果休眠时间设定大于屏保时间，则先进入屏保，达到休眠时间后再进入休眠

修改后的效果

分析与实现：
1.由于Android原生系统默认进入屏保的条件是充电或插入基座时，但到达休眠时才进入屏保。所以需要把进入屏保的条件修改为任何时候都能进入。

frameworks\base\core\res\res\values\config.xml
把"config_dreamsEnabledOnBattery"修改为true即实现任何时候都能进入屏保

<!-- Are we allowed to dream while not plugged in? -->
    <bool name="config_dreamsEnabledOnBattery">true</bool>

2.在设置–>显示–>屏保中添加“屏幕互保”选项开关
2.1添加系统自定义全局变量，用来保存“屏幕互保”选项开关的状态值
frameworks/base/core/java/android/provider/Settings.java

+        /** @hide */
+        public static final String SCREENSAVER_SWITCH= "seewo.screensavers.switch";

public static final class Secure extends NameValueTable {
/** @hide */
public static final String SCREENSAVER_TIMEOUT= “seewo.screensavers.timeout”;

2.2 vendor/mediatek/proprietary/packages/apps/MtkSettings/res/values-zh-rCN/strings.xml

<string name="screensaver_switch_title">屏幕互保</string>

2.3 vendor/mediatek/proprietary/packages/apps/MtkSettings/res/xml/dream_fragment_overview.xml

<PreferenceScreen
     xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:settings="http://schemas.android.com/apk/res-auto"
     android:key="dream_overview_screen"
<SwitchPreference
        android:key="screensaver_switch"
        android:title="@string/screensaver_switch_title"/>

2.4添加vendor/mediatek/proprietary/packages/apps/MtkSettings/src/com/android/settings/dream/ScreenSaverSwitchPreferenceController.java文件

/*
 * Copyright (C) 2017 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.dream;

import static android.provider.Settings.Secure.SCREENSAVER_SWITCH;

import android.content.Context;
import android.provider.Settings;
import android.support.v14.preference.SwitchPreference;
import android.support.v7.preference.Preference;

import com.android.settings.R;
import com.android.settings.core.PreferenceControllerMixin;
import com.android.settingslib.core.AbstractPreferenceController;

public class ScreenSaverSwitchPreferenceController extends AbstractPreferenceController
        implements PreferenceControllerMixin, Preference.OnPreferenceChangeListener {

    private static final String KEY_SCREENSAVER_SWITCH_NAME = "screensaver_switch";

    public ScreenSaverSwitchPreferenceController(Context context) {
        super(context);
    }

    @Override
    public boolean isAvailable() {
        return true;
    }

    @Override
    public String getPreferenceKey() {
        return KEY_SCREENSAVER_SWITCH_NAME;
    }

    @Override
    public boolean onPreferenceChange(Preference preference, Object newValue) {
        boolean value = (Boolean) newValue;
        Settings.Secure.putInt(mContext.getContentResolver(),
                KEY_SCREENSAVER_SWITCH_NAME, value ? 1 : 0);
        if (value) {
            Settings.Secure.putInt(mContext.getContentResolver(), Settings.Secure.SCREENSAVER_SWITCH,
                    1);
        } else {
            Settings.Secure.putInt(mContext.getContentResolver(), Settings.Secure.SCREENSAV