prejoin: fixup mobile

Avoid importing components, which are not yet implemented on mobile.

react/features/prejoin/functions.js

  * @returns {Object}
  */
 export function getAudioTrack(state: Object): Object {
-    return state['features/prejoin'].audioTrack;
+    return state['features/prejoin']?.audioTrack;
 }
 
 /**
  * @returns {Object}
  */
 export function getContentSharingTrack(state: Object): Object {
-    return state['features/prejoin'].contentSharingTrack;
+    return state['features/prejoin']?.contentSharingTrack;
 }
 
 /**
  * @returns {string}
  */
 export function getDeviceStatusText(state: Object): string {
-    return state['features/prejoin'].deviceStatusText;
+    return state['features/prejoin']?.deviceStatusText;
 }
 
 /**
  * @returns {string}
  */
 export function getDeviceStatusType(state: Object): string {
-    return state['features/prejoin'].deviceStatusType;
+    return state['features/prejoin']?.deviceStatusType;
 }
 
 /**
  * @returns {Object}
  */
 export function getVideoTrack(state: Object): Object {
-    return state['features/prejoin'].videoTrack;
+    return state['features/prejoin']?.videoTrack;
 }
 
 /**
  * @returns {boolean}
  */
 export function isPrejoinAudioMuted(state: Object): boolean {
-    return state['features/prejoin'].audioMuted;
+    return state['features/prejoin']?.audioMuted;
 }
 
 /**
  * @returns {boolean}
  */
 export function isPrejoinVideoMuted(state: Object): boolean {
-    return state['features/prejoin'].videoMuted;
+    return state['features/prejoin']?.videoMuted;
 }
 
 /**
  * @returns {string}
  */
 export function getRawError(state: Object): string {
-    return state['features/prejoin'].rawError;
+    return state['features/prejoin']?.rawError;
 }
 
 /**
  * @returns {boolean}
  */
 export function isAudioDisabled(state: Object): Object {
-    return state['features/prejoin'].audioDisabled;
+    return state['features/prejoin']?.audioDisabled;
 }
 
 /**
  * @returns {boolean}
  */
 export function isPrejoinVideoDisabled(state: Object): Object {
-    return state['features/prejoin'].videoDisabled;
+    return state['features/prejoin']?.videoDisabled;
 }
 
 /**
  * @returns {boolean}
  */
 export function isJoinByPhoneDialogVisible(state: Object): boolean {
-    return state['features/prejoin'].showJoinByPhoneDialog;
+    return state['features/prejoin']?.showJoinByPhoneDialog;
 }
 
 /**
  * @returns {boolean}
  */
 export function isPrejoinPageVisible(state: Object): boolean {
-    return isPrejoinPageEnabled(state) && state['features/prejoin'].showPrejoin;
+    return isPrejoinPageEnabled(state) && state['features/prejoin']?.showPrejoin;
 }

react/features/toolbox/components/AudioMuteButton.js

     isPrejoinAudioMuted,
     isAudioDisabled,
     isPrejoinPageVisible
-} from '../../prejoin';
+} from '../../prejoin/functions';
 import { muteLocal } from '../../remote-video-menu/actions';
 
 declare var APP: Object;

react/features/toolbox/components/VideoMuteButton.js

     isPrejoinPageVisible,
     isPrejoinVideoDisabled,
     isPrejoinVideoMuted
-} from '../../prejoin';
+} from '../../prejoin/functions';
 import UIEvents from '../../../../service/UI/UIEvents';
 
 declare var APP: Object;