[RN] Update audio-only icon

Match the one on the web.

react/features/toolbox/components/Toolbox.native.js

                     style = { style }
                     underlayColor = { underlayColor } />
                 <ToolbarButton
-                    iconName = 'hangup'
-                    iconStyle = { styles.toggleAudioOnlyIcon }
+                    iconName = 'visibility'
+                    iconStyle = { iconStyle }
                     onClick = { this.props._onToggleAudioOnly }
                     style = { style }
                     underlayColor = { underlayColor } />

react/features/toolbox/components/styles.js

      */
     secondaryToolbarButtonIcon,
 
-    /**
-     * The icon style of the toolbar button in {@link #secondaryToolbar} which
-     * toggles the audio-only mode of the current conference.
-     */
-    toggleAudioOnlyIcon: {
-        ...secondaryToolbarButtonIcon,
-        transform: [ { rotate: '135deg' } ]
-    },
-
     /**
      * The style of the root/top-level {@link Container} of {@link Toolbox}
      * which contains {@link Toolbar}s.