ARI: Fix implicit answer when playback is initiated on unanswered channel

When issuing a POST /channels/{channel_id}/play on a channel that is not
yet answered, ARI is supposed to:
* Queue up an AST_CONTROL_PROGRESS on the channel
* Start up the playback of the media

Instead, we sneak an answer on the channel right before starting playing media.

This is due to ARI's usage of control_streamfile. This function implicitly
answers the channel (and doesn't give ARI the option to stop it). The answering
of the channel here is probably unnecessary:
* app_voicemail, by far the biggest consumer of this function, always answers
  the channels anyway
* control stream file (in res_agi) and ControlPlayback probably shouldn't be
  implicitly answering the channel. Answering should not be tied directly to
  playing back media.

As it turns out, the answering of the channel here is pretty old:
356042    twilson       if (ast_channel_state(chan) != AST_STATE_UP) {
  3087      anthm               res = ast_answer(chan);
180259   tilghman       }

(As in, ancient?)

Note that others ran into this problem and commented about it on various
mailing lists.

Review: https://reviewboard.asterisk.org/r/3907/

ASTERISK-24229 #close
Reported by: Matt Jordan
........

Merged revisions 421695 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@421696 f38db490-d61c-443f-a65b-d21fe96a405b