Demuxer port

[juanrubio]

The container demuxer class contains a pcm and a raw video ports that need to support the usual indexes. In addition, the following two indexes also need to be supported:
OMX_IndexParamNumAvailableStreams (r)
OMX_IndexParamActiveStream (r/w)

With the above requirements, create a generic demuxer port that can be instantiated for either audio or video domains.