bridge_softmix.c: Report not talking immediately when muted.

Currently in app_confbridge if someone mutes a channel while that channel
is talking, the talk detection code is suspended while the channel is
muted.  As far an an external observer is concerned, the muted channel's
talk status is still "talking" even though the channel is not contributing
audio to the conference bridge.  When the channel is later unmuted, it
takes the usual 'dsp_silence_threshold' option time to clear the talking
status even though the channel may have stopped talking while the channel
was muted.

* In bridge_softmix.c, clear the talking status and report talking stopped
if the channel was talking when the channel is muted.  When the channel is
unmuted and the channel is still talking then report the channel as
talking since it is contributing audio to the bridge again.

ASTERISK-27647

Change-Id: Ie4fdbc05a0bc7343c2972bab012e2567917b3d4e