android:persistent

Whether or not the application should remain running at all times — "true" if it should, and "false" if not. The default value is "false". Applications should not normally set this flag; persistence mode is intended only for certain system applications.

只对system image内部的system app起作用。

http://stackoverflow.com/questions/3529834/how-to-keep-a-android-app-always-running#comment6799178_5811609

【移动开发】android:persistent