SIPCall/ICE - Share unique ICE instance bettween subcalls

Currently, when making an outgoing call to a Jami account with multiple registered devices, a new ice media instance is created for each subcall. This is not necessary, because all ice instances will be similar until the peer answers (and provide its candidates). But when the answer is received from the peer, all ice instances are destroyed except the instance of the successful subcall, which will be used to negotiate the media path. Instead, only one instance should be created for the parent call, and shared amongst the attached subcalls.