Filter internal channels out of bridge enter/leave message handling

Some channels exist merely as an implementation detail in Asterisk, such as
ConfBridge's announcer/recorder channels. These channels should never be
exposed to the outside world, or to interfaces that report on Asterisk. We
already filter out such channels in snapshot processing; however, we failed to
filter out bridge related messages that involved these channels.

This patch filters out bridge related messages that are for such channels. This
prevents a spurious WARNING message from being displayed when those channels
move in and out of bridges.
........

Merged revisions 399146 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@399147 f38db490-d61c-443f-a65b-d21fe96a405b