Update heartbeat interval to a fixed 300 seconds

Author: ianmcorvidae

meshtastic/mesh_interface.py

@@ -715,9 +715,8 @@ def _startHeartbeat(self):
 
         def callback():
             self.heartbeatTimer = None
-            prefs = self.localNode.localConfig
-            i = prefs.power.ls_secs / 2
-            logging.debug(f"Sending heartbeat, interval {i}")
+            i = 300
+            logging.debug(f"Sending heartbeat, interval {i} seconds")
             if i != 0:
                 self.heartbeatTimer = threading.Timer(i, callback)
                 self.heartbeatTimer.start()