Refactoring of the HeartBeat code

Refactoring the code from the vehicle HeartBeat  receiver to a new class. The steps will be:
- [x] Extract to a new class
- [x] Make it extend the DroneVariable class
- [x] Move it to the drone.variables package
- [x] ~~Change to an ```Executors.newSingleThreadScheduledExecutor``` instead of a handler (as discussed at #444)~~
- [x] Make use of the DroneEvents structure.
- [x] Test it with a real connection to see if we broke anything