ExternalMedia: Change return object from ExternalMedia to Channel

When we created the External Media addition to ARI we created an
ExternalMedia object to be returned from the channels/externalMedia
REST endpoint.  This object contained the channel object that was
created plus local_address and local_port attributes (which are
also in the Channel variables).  At the time, we thought that
creating an ExternalMedia object would give us more flexibility
in the future but as we created the sample speech to text
application, we discovered that it doesn't work so well with ARI
client libraries that a) don't have the ExternalMedia object
defined and/or b) can't promote the embedded channel structure
to a first-class Channel object.

This change causes the channels/externalMedia REST endpoint to
return a Channel object (like channels/create and channels/originate)
instead of the ExternalMedia object.

Change-Id: If280094debd35102cf21e0a31a5e0846fec14af9