Create feature-downloads (Non UI)

[Amejia481]

Breaking in small parts this issue #459

• Subscribe to downloads in the currently selected session (I assume we do not need to subscribe to it in all Sessions?)

• Consume Download objects and pass the data to Androids Download Manager

• Call back into the app to notify if a download is done

┆Issue is synchronized with this Jira Task

[pocmo]

@csadilek This is probably another use case for #962 (always tracking the current session) :)