[image_picker] Removes use of PHAsset on IOS 14+

[WenHaozhan]

Original PR #8020
Fixes issue flutter/flutter#90373

Currently on IOS 14+, images are picked using PHPickerViewController which does not need photo permissions and also gets the full image metadata regardless of requestFullMetadata. However, it currently retrieves the metadata using PHAsset which does require permission and causes the gallery opening twice issue. Another issue is that an error is thrown when permission is denied even if none are required.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[loic-sharma]

@WenHaozhan Hey there, thanks for updating this! I think this PR looks good, but I'm new to this package and I'd like to confirm with my co-workers. It's the holidays in the US, so I will likely loop back next week. I apologize for the delay

[loic-sharma]

Looks good to me - thanks for the contribution!