fix(audio): acquire streams before negotiation when peer is answerer

When a sendrecv peer acts as the answerer, gUM is only called _after_
the remote offer is received. This is fine, but the error handling runs
different in that scenario in a way that eventual gUM errors are treated
as negotiation errors, leading to inconsistencies when surfacing the
error to end users.

If a peer is acting as answerer and is a transceiver, acquire the local
streams _before_ actual negotiation so that gUM errors are surfaced
correctly (and we spare uneeded negotiation steps).

bigbluebutton-html5/imports/ui/services/bbb-webrtc-sfu/audio-broker.js

@@ -97,11 +97,20 @@ class AudioBroker extends BaseBroker {
         this.webRtcPeer.peerConnection.onconnectionstatechange = this.handleConnectionStateChange.bind(this);
 
         if (this.offering) {
+          // We are the offerer
           this.webRtcPeer.generateOffer()
             .then(this.sendStartReq.bind(this))
             .catch(this._handleOfferGenerationFailure.bind(this));
-        } else {
+        } else if (peerRole === 'recvonly') {
+          // We are the answerer and we are only listening, so we don't need
+          // to acquire local media
           this.sendStartReq();
+        } else {
+          // We are the answerer and we are sending audio, so we need to acquire
+          // local media before sending the start request
+          this.webRtcPeer.mediaStreamFactory()
+            .then(() => { this.sendStartReq(); })
+            .catch(this._handleOfferGenerationFailure.bind(this));
         }
 
         resolve();