mute in conference: rework mute/un-mute of local host

When a call is added to a conference, the control of the mute/un-mute
state of the media of the call is taken over by the conference, and
the mute state of the participating calls will be controlled by the
state of the local host set in the conference, which basically consists
of attaching/detaching the source to/from the mixer. Currently the local
host mute state might not be correctly initialized, leading to
inconsistent mute states.
The proposed changes will correctly set the local host state according
to the initial mute states of each call joining the conference.

Gitlab: #576

Change-Id: I0a746aae82da57222cc7ff91c2e39a1a2bbaff8e