Refactor to follow MVVM architecture

[kvizcarra]

Refactor the project to follow the MVVM architecture pattern. This reorganizing will be important for unit testing in a future issue.

View

• Activity
• Fragment

ViewModel

• Android Architecture ViewModel

Model

• Repository
• Android Architecture Room and DAO

The View should observe LiveData returned from the ViewModel for any data changes. The ViewModel will expose CRUD methods for any class, in this case the View, to call. The ViewModel will relay CRUD calls to a repository in the Model layer. The repository talks to Room through DAOs and returns LiveData.

Interfaces should be used for communication between classes.