com.android.server.SystemService
com.android.server.SystemServiceManager
android 5.1开始，系统服务如PowerManagerService继承自SystemService
继承SystemService的好处有如下：
1. 实现onBootPhase(int phase)，配合SystemServiceManager可以在启动各个阶段处理对应事件
public static final int PHASE_WAIT_FOR_DEFAULT_DISPLAY = 100;
public static final int PHASE_LOCK_SETTINGS_READY = 480;
public static final int PHASE_SYSTEM_SERVICES_READY = 500;
public static final int PHASE_ACTIVITY_MANAGER_READY = 550;
public static final int PHASE_THIRD_PARTY_APPS_CAN_START = 600;
public static final int PHASE_BOOT_COMPLETED = 1000;
2. 增加了LocalServices
这样SystemServer进程里各个服务调用就不用通过Binder，可以通过getLocalService获取
比如PowerManagerService中添加LocalService：
publishLocalService(PowerManagerInternal.class, new LocalService());
接下来就可以在 ActivityManagerService 调用PowerManagerService里的接口：
mLocalPowerManager = LocalServices.getService(PowerManagerInternal.class);
