conference: add participant list not updated during the call/conference

assigned to @aberaud and @sblin

unassigned @sblin and @aberaud

added good-first-issue label

Scenario:

1. Add an offline contact
2. In a call click on add-participants
3. While the popup is shown, the contact comes online

(expected) we should see the contact

(current) the list is not refreshed, we need to re-click on add participant

closed

mentioned in commit 5f7bf66b

Is this issue actually related: jami-client-android#1129 ?