1.概述

在12.0的系统rom定制化开发中，在第三方app开发中，需要开启无障碍服务功能，就不需要在代码中开启无障碍服务了，
为了简便就需要在系统中开启无障碍服务，来实现开启无障碍服务功能

2. 默认开启无障碍服务权限和打开默认apk无障碍服务核心代码

frameworks/base/core/java/android/provider/Settings.javaframeworks/base/packages/SettingsProvider/src/com/android/providers/settings/DatabaseHelper.java

3. 默认开启无障碍服务权限和打开默认apk无障碍服务的功能分析和实现
   3.1 Settings.java关于无障碍服务的定义

          public final class Settings {     /*** Indicates which clock face to show on lock screen and AOD while docked.* @hide*/public static final String DOCKED_CLOCK_FACE = "docked_clock_face";/*** Set by the system to track if the user needs to see the call to action for* the lockscreen notification policy.* @hide