Executing dialplan functions from either AMI or ARI by getting a variable
could place the channel into autoservice.  However, these user interface
threads do not handle the channel's media so we wind up with two threads
attempting to handle the media.

There can be one and only one thread handling a channel's media at a time.
Otherwise, we don't know which thread is going to handle the media frames.

ASTERISK-27625

Change-Id: If2dc94ce15ddabf923ed1e2a65ea0ef56e013e49