app_stream_echo: Fix state of added streams.

When stream support was added to Asterisk the stream state
was used inconsistently, resulting in odd behavior. This
was then standardized to be the state of a stream from the
perspective of Asterisk.

This change updates the StreamEcho dialplan application
to use the correct state, send only, since we are only
sending to the endpoint and not expecting them to send us
multiple video streams.

ASTERISK-28954

Change-Id: I35bfd533ef1184ffe62586b22bbd253c82872a56