The current state of the media stream.
'invalidStreamState' - None of the following.
'idle' - Initialization state. The stream
is yet to be forked.
'adding' - The stream is getting forked.
'deleting' - The stream is being deleted from
the call.
'changing' - The stream is in the process of
changing its state.
'active' - The stream is now active and
communication is on.
'dead' - The stream has been cleared.