[todo-mvp-clean] Applying Dependency rule

I noticed the sample is not following the dependency rule proposed by the linked article Clean Architecture.I.e. [GetTasks](https://github.com/googlesamples/android-architecture/blob/todo-mvp-clean/todoapp/app/src/main/java/com/example/android/architecture/blueprints/todoapp/tasks/domain/usecase/GetTasks.java) uses the Task Repository, which belongs to the data package. I would argue it is a style common when writing a 3-layer architecture where the TaskRepository is used as a Port to access the data layer.

In clean architecture the depencendy rule would force us to have a interface inside the domain layer which is implemented by some class in the data layer. As it is very common to have some kind of persistence, we could say we break the rule for convinience? How do we apply the dependency rule to other cases where domain logic wants to talk to the outside world i.e. Messaging or Sharing information.

What do you think? How did you manage this in practice ?

Start with todoapp/app/src/main/java/com/example/android/architecture/blueprints/todoapp/tasks/domain/usecase/GetTasks.java and the linked Clean Architecture article. The issue is a design discussion rather than a defined change, so first establish the desired dependency rule and affected outside-world interactions; done criteria are not specified.

Written by the indexing model from the issue text.