bridge_native_rtp: Fix audio issues when moving from remote bridge to softmix

bridge_native_rtp: Fix audio issues when moving from remote bridge to softmix

When a native RTP bridge that is remotely bridging its participants switches
to a softmix bridge, it may not properly re-INVITE the media for one or both
participants back to Asterisk. This is due to the current bridge_native_rtp
code only re-INVITEs if it believes the channel will survive the bridge
operation. Currently, that code is failing, as it expects the channels to
have a soft hangup flag set on it indicating that a redirect has occurred
or that the channel is going to leave the bridge. (The code did not take into
account a smart bridge operation).

This patch also renames a few things to be more reflective of the underlying
types.

Review: https://reviewboard.asterisk.org/r/3997/

ASTERISK-24327 #close

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@425760 65c4cc65-6c06-0410-ace0-fbb531ad65f3