app_confbridge: Don't play leader leaving prompt if no one will hear it

Consider the following:
- A marked user in a conference
- One or more end_marked only users in the conference

When the marked users leaves, we will be in the conf_state_multi_marked state.
This currently will traverse the users, kicking out any who have the end_marked
flags. When they are kicked, a full ast_bridge_remove is immediately called on
the channels. At this time, we also unilaterally set the need_prompt flag.

When the need_prompt flag is set, we then playback a sound to the bridge
informing everyone that the leader has left; however, no one is left in the
bridge. This causes some odd behaviour for the end_marked users - they are
stuck waiting for the bridge to be unlocked. This results in them waiting for
5 or 6 seconds of dead air before hearing that they've been kicked.

Unfortunately, we do have to keep the bridge locked while we're playing back
the 'leader-has-left' prompt. If there are any wait_marked users in the
conference, this behaviour can't be easily changed - but we do make the case
of the end_marked users better with this patch.

Review: https://reviewboard.asterisk.org/r/4184/

ASTERISK-24522 #close
Reported by: Matt Jordan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@428077 f38db490-d61c-443f-a65b-d21fe96a405b