ice: share ice instance between 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 will be created for the parent call, and
shared amongst the attached subcalls.

Gitlab: #619

Change-Id: I001c27f69d21b3ea640b189aea401e43c3f6bdef