[RN] Handle presence

Up until now, mobile was oblivious to participants' presence state. Presence
state handling is required (probably, amongst other things) for "call flows".
So, let's add it! This is done by gathering the presence state when a
participant first joins, and handling subsequent changes.

react/features/base/conference/actions.js

@@ -15,6 +15,7 @@ import {
     participantConnectionStatusChanged,
     participantJoined,
     participantLeft,
+    participantPresenceChanged,
     participantRoleChanged,
     participantUpdated
 } from '../participants';
@@ -146,6 +147,7 @@ function _addConferenceListeners(conference, dispatch) {
             conference,
             id,
             name: user.getDisplayName(),
+            presence: user.getStatus(),
             role: user.getRole()
         })));
     conference.on(
@@ -155,6 +157,9 @@ function _addConferenceListeners(conference, dispatch) {
     conference.on(
         JitsiConferenceEvents.USER_ROLE_CHANGED,
         (...args) => dispatch(participantRoleChanged(...args)));
+    conference.on(
+        JitsiConferenceEvents.USER_STATUS_CHANGED,
+        (...args) => dispatch(participantPresenceChanged(...args)));
 
     conference.addCommandListener(
         AVATAR_ID_COMMAND,