Fixed CameraException on Android when disposing (#257)

lib/src/qr_code_scanner.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:io';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
@@ -311,7 +312,7 @@ class QRViewController {
 
   /// Stops the camera and disposes the barcode stream.
   void dispose() {
-    stopCamera();
+    if (Platform.isIOS) stopCamera();
     _scanUpdateController.close();
   }