ICE: add srflx candidates in answer

ICE srflx candidates are not added when answering a call. This
could cause media ICE negotiation to fail if the local endpoint
does not have TURN and UPNP enabled, and peer only has TURN enabled.
Now, srflx candidates are added in the answer as done in the offer.

Gitlab: #607

Change-Id: I5af3e24e85846f518cf4f746d31c067dcf57ff15