[quick_actions] Fix UIApplicationShortcutItem availability and pod lint warnings

[jmagman]

Description

Fix quick_actions UIApplicationShortcutItem availability warnings, slightly cleaned up FLTQuickActionsPlugin (though I'm not convinced this plugin is working).
Also fix pod lib lint warnings:

 -> quick_actions (0.0.1)
    - WARN  | license: Missing license type.
    - WARN  | description: The description is equal to the summary.
    - WARN  | [iOS] keys: Missing primary key for `source` attribute. The acceptable ones are: `git, hg, http, svn`.
    - WARN  | [iOS] xcodebuild:  /Users/m/Projects/plugins/packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m:44:35: warning: 'UIApplicationShortcutItem' is only available on iOS 9.0 or newer [-Wunguarded-availability]
    - WARN  | [iOS] xcodebuild:  /Users/m/Projects/plugins/packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m:63:8: warning: 'UIApplicationShortcutItem' is only available on iOS 9.0 or newer [-Wunguarded-availability]
    - WARN  | [iOS] xcodebuild:  /Users/m/Projects/plugins/packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m:29:39: warning: 'setShortcutItems:' is only available on iOS 9.0 or newer [-Wunguarded-availability]
    - WARN  | [iOS] xcodebuild:  /Users/m/Projects/plugins/packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m:56:5: warning: 'UIApplicationShortcutItem' is only available on iOS 9.0 or newer [-Wunguarded-availability]
    - WARN  | [iOS] xcodebuild:  /Users/m/Projects/plugins/packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m:60:37: warning: 'setShortcutItems:' is only available on iOS 9.0 or newer [-Wunguarded-availability]
    - WARN  | [iOS] xcodebuild:  /Users/m/Projects/plugins/packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m:64:3: warning: 'UIApplicationShortcutIcon' is only available on iOS 9.0 or newer [-Wunguarded-availability]
    - WARN  | [iOS] xcodebuild:  /Users/m/Projects/plugins/packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m:67:14: warning: 'UIApplicationShortcutIcon' is only available on iOS 9.0 or newer [-Wunguarded-availability]
    - WARN  | [iOS] xcodebuild:  /Users/m/Projects/plugins/packages/quick_actions/ios/Classes/FLTQuickActionsPlugin.m:69:12: warning: 'UIApplicationShortcutItem' is only available on iOS 9.0 or newer [-Wunguarded-availability]
...
[!] quick_actions did not pass validation, due to 11 warnings (but you can use `--allow-warnings` to ignore them).

Related Issues

flutter/flutter#55245
Dependency to merge https://github.com/flutter/plugin_tools/pull/97

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]).
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Breaking 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.

[cyanglaz]

LGTM!