[shared_preferences] Fix JSON parsing issue with _decodeValue

[Dan-Crane]

[shared_preferences] Fix JSON parsing issue with getAllWithParameters

this is a restored PR #5813

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.
  https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changelog-style
  [exempt from CHANGELOG changes]: https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changelog
  [test-exempt]: https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#tests