fix(mobile/call-integration): cleanup if leave takes too long

The conference disconnection process is asynchronous which means there's
no guarantee that there will be CONFERENCE_LEFT event for the old
conference, before the next conference is joined. Because of that we can
end up with two simultaneous calls on the native side which is not
always supported. End the call on CONFERENCE_WILL_LEAVE to fix this
corner case.

react/features/mobile/call-integration/middleware.js

 import { APP_WILL_MOUNT, APP_WILL_UNMOUNT } from '../../base/app';
 import {
     CONFERENCE_FAILED,
+    CONFERENCE_JOINED,
     CONFERENCE_LEFT,
     CONFERENCE_WILL_JOIN,
-    CONFERENCE_JOINED,
+    CONFERENCE_WILL_LEAVE,
     SET_AUDIO_ONLY,
     getConferenceName,
     getCurrentConference
     case CONFERENCE_JOINED:
         return _conferenceJoined(store, next, action);
 
+    // If a conference is being left in a graceful manner then
+    // the CONFERENCE_WILL_LEAVE fires as soon as the conference starts
+    // disconnecting. We need to destroy the call on the native side as soon
+    // as possible, because the disconnection process is asynchronous and
+    // Android not always supports two simultaneous calls at the same time
+    // (even though it should according to the spec).
     case CONFERENCE_LEFT:
+    case CONFERENCE_WILL_LEAVE:
         return _conferenceLeft(store, next, action);
 
     case CONFERENCE_WILL_JOIN:
         const { callUUID } = action.conference;
 
         if (callUUID) {
+            delete action.conference.callUUID;
             CallIntegration.reportCallFailed(callUUID);
         }
     }
     const { callUUID } = action.conference;
 
     if (callUUID) {
+        delete action.conference.callUUID;
         CallIntegration.endCall(callUUID);
     }