Return on_since only when its available and the device is on (#48)

* moves on_since property to smartdevice class, as it is not plug only
* returns None if the value is not available (some bulbs), or if the device is off

kasa/smartplug.py

@@ -1,5 +1,4 @@
 """Module for plugs."""
-import datetime
 import logging
 from typing import Any, Dict
 
@@ -81,18 +80,6 @@ class SmartPlug(SmartDevice):
         await self._query_helper("system", "set_led_off", {"off": int(not state)})
         await self.update()
 
-    @property  # type: ignore
-    @requires_update
-    def on_since(self) -> datetime.datetime:
-        """Return pretty-printed on-time.
-
-        :return: datetime for on since
-        :rtype: datetime
-        """
-        on_time = self.sys_info["on_time"]
-
-        return datetime.datetime.now() - datetime.timedelta(seconds=on_time)
-
     @property  # type: ignore
     @requires_update
     def state_information(self) -> Dict[str, Any]: