android4.4\frameworks\base\packages\Keyguard\src\com\android\keyguard\KeyguardViewMediator.java
/**
* External apps (like the phone app) can tell us to disable the keygaurd.
*/
private boolean mExternallyEnabled = false;//系统此属性默认是true，本类把所有的mExternallyEnabled 此属性改成false，系统永远就不锁屏了