Fix joining a different call on the same signaling server

The token passed to "startCall" was set when WebRTC was set up. However,
WebRTC is set up only when the signaling server is changed. Therefore,
when joining a call in a different conversation but in the same
signaling server the token used was the one from the first conversation.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>