[quick_actions]refactor into smaller components and 100% unit test coverage

[hellohuanlin]

• Refactor shortcut related logics into a separate FLTShortcutStateManager which manages a list of all shortcut items. The FLTQuickActionsPlugin class does not directly depend on [UIApplication sharedApplication].shortcutItems anymore.
• Also do not expose the launchingShortcutType.

This refactor allows us to reach 100% unit test coverage without exposing private APIs.

List which issues are fixed by this PR. You must list at least one issue.

https://github.com/flutter/flutter/issues/108750

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

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

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