[RN] Disable remote video while in the background

Set the video channel "last N" property to 0, thus making the client not receive
any remote video.

react/features/background/actionTypes.js

 export const _SET_BACKGROUND_VIDEO_MUTED
     = Symbol('_SET_BACKGROUND_VIDEO_MUTED');
 
+/**
+ * The type of redux action which signals that the video channel's lastN value
+ * must be changed.
+ *
+ * {
+ *      type: _SET_LASTN,
+ *      lastN: boolean
+ * }
+ *
+ * @protected
+ */
+export const _SET_LASTN
+    = Symbol('_SET_LASTN');
+
+
 /**
  * The type of redux action which signals that the app state has changed (in
  * terms of execution mode). The app state can be one of 'active', 'inactive',

react/features/background/actions.js

 import {
     _SET_APP_STATE_LISTENER,
     _SET_BACKGROUND_VIDEO_MUTED,
+    _SET_LASTN,
     APP_STATE_CHANGED
 } from './actionTypes';
 
  */
 export function _setBackgroundVideoMuted(muted: boolean) {
     return (dispatch, getState) => {
+        // Disable remote video when we mute by setting lastN to 0.
+        // Skip it if the conference is in audio only mode, as it's
+        // already configured to have no video.
+        const { audioOnly } = getState()['features/base/conference'];
+
+        if (!audioOnly) {
+            const { config } = getState()['features/base/lib-jitsi-meet'];
+            const defaultLastN
+                = typeof config.channelLastN === 'undefined'
+                ? -1 : config.channelLastN;
+
+            dispatch({
+                type: _SET_LASTN,
+                lastN: muted ? 0 : defaultLastN
+            });
+        }
+
         if (muted) {
             const mediaState = getState()['features/base/media'];
 
                 return;
             }
         } else {
-            const bgState = getState()['features/background'];
+            const { videoMuted } = getState()['features/background'];
 
-            if (!bgState.videoMuted) {
+            if (!videoMuted) {
                 // We didn't mute video, do nothing.
                 return;
             }
         }
 
-        // Remember that video was muted due to the app going to the background
-        // vs user's choice.
+        // Remember that local video was muted due to the app going to the
+        // background vs user's choice.
         dispatch({
             type: _SET_BACKGROUND_VIDEO_MUTED,
             muted

react/features/background/middleware.js

 } from './actions';
 import {
     _SET_APP_STATE_LISTENER,
+    _SET_LASTN,
     APP_STATE_CHANGED
 } from './actionTypes';
 
         break;
     }
 
+    case _SET_LASTN: {
+        const { conference } = store.getState()['features/base/conference'];
+
+        if (conference) {
+            try {
+                conference.setLastN(action.lastN);
+            } catch (err) {
+                console.warn(`Error setting lastN: ${err}`);
+            }
+        }
+
+        break;
+    }
+
     case APP_STATE_CHANGED:
         _appStateChanged(store.dispatch, action.appState);
         break;