Changing the way we manage the callback handle id. Since same instance is used from plugin registry for all plugin callbacks and plugin callbacks are sort of async after first call to 'call' method in a plugin, it's important that callbackHandle lives in intent that an instance varaible of class; as the latter can potentially be overridden.
Merge pull request #98 from priyaaank/plugin_callback_handle_fix

Alert Plugin uses a better pattern to manage callback handle id