[image_picker] Rename iOS class according to the official Objective-C naming convention.

[cyanglaz]

Description

Certain classes/types in the image picker iOS implementation are not following the official naming convention described here https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingBasics.html#//apple_ref/doc/uid/20001281-1002226-BBCJECED.
(2 or 3 letters prefix before a class/type name to avoid conflicts between other frameworks)

This PR added FLT to all the Classes, Enums, Typedefs, and Constants in this plugin.

Related Issues

N/A

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[collinjackson]

lgtm