Fix iOS button enabled for no preview

[leontobias]

I have updated the behavior where the asset picker for iOS would still show the confirm button in the bottom app bar even if isSingleAssetMode == true and isPreviewEnabled == false.

I assume this to be a bug as:

1. Android is behaving different (as expected)
2. Inside appleOSLayout I assume the isAppleOS check is unneeded

If suitable, please let me know whether this needs a changelog entry and which formatting requirements this has.

closes #377

[AlexV525]

Thanks for fixing this. Something should be indicated before the review.

• The behavior should be the same as Android, only with the different position.
• Tests should be added, and we had tests against the confirm button on Android already so it'll be easy to write a new one.
• CHANGELOG.md, pubspec.yaml and example.yaml should be updated correspondingly.

[AlexV525]

@all-contributors Add @leonpesdk for bugs.

[allcontributors]

@AlexV525

I've put up a pull request to add @leonpesdk! 🎉