Fix launchUrl Android returning null

* Fix parsing issue, rename key from launchURL to launchUrl

Author: Jeasmine

android/src/main/java/com/onesignal/flutter/OneSignalSerializer.java

@@ -152,7 +152,7 @@ static HashMap<String, Object> convertNotificationToMap(OSNotification notificat
         if (notification.getSmallIconAccentColor() != null)
             hash.put("smallIconAccentColor", notification.getSmallIconAccentColor());
         if (notification.getLaunchURL() != null)
-            hash.put("launchURL", notification.getLaunchURL());
+            hash.put("launchUrl", notification.getLaunchURL());
         if (notification.getSound() != null)
             hash.put("sound", notification.getSound());
         if (notification.getLedColor() != null)