Add ability to init and teardown classes when user scoped services change

[chris-mitchell]

Consumers often want the ability to register for events when user scoped services are created. Some examples of this might be uploading user files, notifications, etc. We currently have a callback system with teardown, but we can improve this.

We could use Dagger's IntoSet to allow components to register classes to instantiate and call when the scoped services are created. Similarly, we could do this for teardown classes as well.

[chris-mitchell]

This is similar to https://github.com/dropbox/kaiken/issues/29