Add mysensors device tracker and platform discovery (#5781)

* Add mysensors device_tracker platform

* Add discovery of device_tracker platforms

* Enable discovery of device_tracker platforms that are not
  DeviceScanner.
* Update signature of setup_scanner function in all affected platforms.
* Add test.
* Use discovery for mysensors device_tracker platform.

* Remove gps accuracy

* Small change to core like schema

* fix depency

homeassistant/components/device_tracker/demo.py

@@ -4,7 +4,7 @@ import random
 from homeassistant.components.device_tracker import DOMAIN
 
 
-def setup_scanner(hass, config, see):
+def setup_scanner(hass, config, see, discovery_info=None):
     """Setup the demo tracker."""
     def offset():
         """Return random offset."""