Replace __ast_answer with ast_raw_answer in app_control_answer

Replace __ast_answer with ast_raw_answer in app_control_answer

While load testing an ARI application, I noticed asterisk was returning HTTP 500
internal server errors on channels/:id/answer.  After talking to #asterisk-dev,
the issue appeared to be a lack of media flowing after __ast_answer() was
called.  So now, we call ast_raw_answer instead and no longer wait for media.

ASTERISK-23758 #close
Review: https://reviewboard.asterisk.org/r/3549/
........

Merged revisions 414195 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@414196 65c4cc65-6c06-0410-ace0-fbb531ad65f3