use base class methods for disconnected event

src/connection/ble_connection.js

@@ -44,7 +44,7 @@ class BleConnection extends Connection {
 
         // listen for ble disconnect
         this.bleDevice.addEventListener("gattserverdisconnected", () => {
-            this.emit("disconnected");
+            this.onDisconnected();
         });
 
         // connect to gatt server

src/connection/connection.js

@@ -9,6 +9,10 @@ class Connection extends EventEmitter {
         this.emit("connected");
     }
 
+    onDisconnected() {
+        this.emit("disconnected");
+    }
+
     async close() {
         throw new Error("This method must be implemented by the subclass.");
     }

src/connection/serial_connection.js

@@ -17,7 +17,7 @@ class SerialConnection extends Connection {
 
         // listen for disconnect
         this.serialPort.addEventListener("disconnect", () => {
-            this.emit("disconnected");
+            this.onDisconnected();
         });
 
         // fire connected callback after constructor has returned