Make Hearbeat available to other modules (#13250)

- When a mobile app goes online/offline we need to change
  Heartbeat interval and restart the schedule.
- We also need to be notified about response status in
  order to show the appropriate offline UI, etc.

Related with Issue #13250 and review https://dev.vaadin.com/review/#/c/3376/

Change-Id: I428501306e37fb8c2ee0ed6022a4c588bd8456db