sipaccount: call SIPStartCall after returning call

Some functions in SIPStartCall can take a long time to return, such
as ICE initialization. This allows the manager to return the callId
in outgoingCall() to the client sooner and so prevents the UI from
freezing.

Change-Id: I87b0c4b5d56f69e087f0117b2b78daa026679aab
Tuleap: #620