android - fix NullPointerExceptions when disabling push notifications

When calling the wrapper functions that do no have a callback parameter, e.g. ```public void disablePushNotificationsOnChannel(String channel, String gcmRegistrationId)```, the wrapped functions get called with ```null``` as the callback parameter.

This leads to a ```NullPointerException``` as the ```successCallback``` and ```errorCallback``` functions get called on the ```callback``` object (that is ```null```) and not the ```cb``` object (not-```null``` object returned by ```getWrappedCallback(callback)```)