title: how to allow another app to access my app's data directory?
body: <p>Suppose I have an app named A with this data directory: com.example.test
  Now I want to make another app named B to modify something in com.example.test.</p>
  
  <p>Of course I know both have to share the same signature. But what else do I have to have?</p>
  
  <p>Basically I am trying to design an app to which users can plug in new components. Any idea?</p>