fix(lastN) simplify lastN calculation when in background (#15018)

There is no point in ever setting lastN to anything other than 0 when in
the background because we are not rendering anything.

When in PiP mode, the app state is not background, which might have
deceived us into adding that check there way back when.

react/features/base/lastn/middleware.ts

@@ -11,7 +11,6 @@ import { SET_AUDIO_ONLY } from '../audio-only/actionTypes';
 import { CONFERENCE_JOINED } from '../conference/actionTypes';
 import { getParticipantById } from '../participants/functions';
 import MiddlewareRegistry from '../redux/MiddlewareRegistry';
-import { isLocalVideoTrackDesktop } from '../tracks/functions';
 
 import { setLastN } from './actions';
 import logger from './logger';
@@ -45,9 +44,7 @@ const _updateLastN = debounce(({ dispatch, getState }: IStore) => {
     // 3. -1 as the default value.
     let lastNSelected = config.startLastN ?? (config.channelLastN ?? -1);
 
-    if (typeof appState !== 'undefined' && appState !== 'active') {
-        lastNSelected = isLocalVideoTrackDesktop(state) ? 1 : 0;
-    } else if (carMode) {
+    if (appState === 'background' || carMode) {
         lastNSelected = 0;
     } else if (audioOnly) {
         const { remoteScreenShares, tileViewEnabled } = state['features/video-layout'];