Java binding: make it available to other swig java binding

* If more than one module is used and SWIG proxy classes need
constructing/using across modules, there is no other option than expose
the getCPtr() of AvTranscoder java binding to public.
* See this link: http://sourceforge.net/p/swig/mailman/swig-
devel/thread/4ED006FD.6020403@fultondesigns.co.uk/