bridge_softmix: Clear "talking" when a channel is put on hold

This patch clears the talking flag from the channel (if already set), and
notifies listeners when that channel is put on hold. Note however, if the
endpoint continues to send audio frames and these are received by the bridge
then that channel will be put back into a "talking" state even though they
are on hold.

ASTERISK-27755 #close

Change-Id: I930e16c4662810f9f02043d69062f88173c5e2ef