createNewMediaItem
Creates a new MediaItem entry in the internal database and returns its Uri. The returned Uri should be used to write the content for the MediaItem into the uri. After the content has been successfully written, markNewMediaItemFinished should be called with the same uri, to mark that the item is ready to be returned at queries.
Note that the item created here will not be returned by any methods of the SOCommMediaClient before markNewMediaItemFinished is called.
In case of an error, call deleteNewMediaItem.
Return
a Single with a Pair containing the Uri for a the mediaItem as well as the OutputStream to write content into this mediaItem. It is the callers responsibility to close this OutputStream.