file transfer: use ICE code in more configurations

+ Public ips was not registered as a candidate. With this patch,
negotiations via UPnP and TURN are made during the ICE negotiation.
So, the fallback with the old code should not be used anymore for
new versions.
+ Both peers negotiate the link at the same time. The speed of the
negotiation is improved.
+ Fix PJ_EBUSY detection when using a TURN socket to transfer a
file.

Change-Id: I74fb313b9628d762a1ab0da05e3a18ea3691cb3d