Move Briar API calls to background thread

While the Briar API calls will now be logically separated from the UI thanks to the view models (!33 (merged) and !34 (merged)), all the calls are still executed on the main (UI) thread.

In the Android app, all those calls are run on a dedicated database thread (see, for example, here).

We should use a similar approach, but we can take advantage of the nice Kotlin coroutines approach, that allows to run coroutines on a different IO thread (pool?).