AGI/Manager: Prevent multiple NewExten events during AGI application changes

AGI applications would trigger NewExten events every time the state of the AGI
application changed. This has historically not been the behavior and this
behavior was introduced with a CDR patch. This patch corrects that.

(closes issue ASTERISK-23390)
Reported by: Benjamin Keith Ford
Review: https://reviewboard.asterisk.org/r/3406/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@411868 f38db490-d61c-443f-a65b-d21fe96a405b