[quick_actions]make QuickActions testable

[The-Redhat]

Description

I used the same approach like in FirebaseMessaging and added a QuickActions.private constructor used for testing with a mock MethodChannel.

Related Issues

Fixes #25887

Checklist

• 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?

• No, this is not a breaking change.

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[The-Redhat]

I signed it!

[googlebot]

CLAs look good, thanks!

[collinjackson]

Thanks for your contribution!

Please update your pull request following the contribution checklist: https://github.com/flutter/plugins/blob/master/.github/PULL_REQUEST_TEMPLATE.md

In particular, it would be helpful if you could contribute a simple unit test for this plugin and update the pubspec/CHANGELOG, and make sure the bots are happy.

[The-Redhat]

Thanks for your answer, since it's my first pr I ask myself what should the unit test consist of ?

[The-Redhat]

I made the requested changes!

[The-Redhat]

Hey, can anybody please look into this pr? Thanks!

[justinmc]

@The-Redhat Can you merge master into this and fix the conflicts? Then we can try to get a code review for this and get things moving.

[The-Redhat]

merged master

[cyanglaz]

Thank you for contributing. Did a quick pass. It looks good overall, left some comments.

[The-Redhat]

I made the requested changes

[cyanglaz]

LGTM! cc @collinjackson Since you initially looked at this, I'd like to also get a 'go' from you before merging this in.

[collinjackson]

We discussed as a team and we'd like unit tests to be written using setMockMethodCallHandler rather than mockito.

Here's an example unit test:
https://github.com/flutter/plugins/blob/master/packages/cloud_functions/test/cloud_functions_test.dart

Could you please update your unit test to use this style instead?

[The-Redhat]

@collinjackson I updated the tests

[The-Redhat]

@collinjackson Can I do something to get this pr merged ?