WorkManager for background tasks

[JoaquimLey]

Sending and receiving might be a good case for this #14

SmsController should maybe use this 🤔

Sources

• https://developer.android.com/topic/libraries/architecture/workmanager/

Other related, in-process background stuff:

• https://developer.android.com/reference/java/util/concurrent/Executors

• https://developer.android.com/training/multiple-threads/run-code