[shared_preferences]Fix : SetState returning future

[berhili098]

This pull request includes a change to the SharedPreferencesDemoState class in the main.dart file. The change improves the way the external counter value is fetched and updated in the state.

• packages/shared_preferences/shared_preferences/example/lib/main.dart: Refactored the _getExternalCounter method to fetch the external counter value before calling setState, ensuring that asynchronous operations are handled correctly.

The issue was :

 FlutterError (setState() callback argument returned a Future.
The setState() method on SharedPreferencesDemoState#1acdd(lifecycle state: created) was called with a closure or method that returned a Future. Maybe it is marked as "async".
Instead of performing asynchronous work inside a call to setState(), first execute the work (without updating the widget state), and then synchronously update the state inside a call to setState().)

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.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[tarrinneal]

@stuartmorgan can you exempt this from versioning? (If you agree that's appropriate)

[stuartmorgan-g]

@stuartmorgan can you exempt this from versioning? (If you agree that's appropriate)

example/lib/main.dart is published on pub.dev, which is why the CI check explicitly doesn't exempt this file in its check. The exemption is for "PRs that only affect unpublished parts of example apps."

That's covered in the PR checklist, which I've restored to the PR description. @berhili098, in the future please don't delete the checklist, as it's there for a reason. I'm marking this as a draft for now; please complete the checklist and then mark it as ready for review once that's done.

[tarrinneal]

I updated the versioning to match the documented style guide

[tarrinneal]

@stuartmorgan for second review

[stuartmorgan-g]

LGTM