Support androidIcon on iOS

[ankit-deligence3112]

Feature proposal

I want to add a lock icon instaed of play/pause in andriod and ios notification control using audio service and just_audio .when i added lock icon like this its working fine in andriod but not showing in ios .how i can show in ios .

if (isPaid)
          MediaControl.custom(
              androidIcon: 'drawable/ic_lock',
              label: 'Locked',
              name: 'Locked',
              extras: <String, dynamic>{'Locked': 1})
### Motivating use case(s)
so that user can add custom icon in ios .

[ryanheise]

I've edited your title to be of a more appropriate length.