[shared_preferences] Shared preferences linux endorsement

[stuartmorgan-g]

Description

Endorses shared_preferences_linux.

Since shared_preferences_linux is pure Dart, this uses manual registration of the Linux implementation. It is similar to the solution used for path_provider, but improves on it by more narrowly tailoring the registration to cases where another registration hasn't already been done, and only checking the first time, thus avoiding the need for a variable to override it.

Updates shared_preferences_linux to depend on path_provider_linux rather than path_provider, to avoid adding unnecessary native plugin dependencies to other platforms.

Also adds a driver test for shared_preference_linux, which I missed in reviewing the initial shared_preferences_linux landing.

Related Issues

Fixes flutter/flutter#41720

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).
• 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?

• 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.

[stuartmorgan-g]

Review notes:

• The linux/example/ files are just generated from the flutter create template without changes.
• The test_driver files are duplicated verbatim from existing platform implementations.

[cyanglaz]

LGTM!

[stuartmorgan-g]

In case there are still dependency conflict issues I tried a version that uses XDG directly to avoid the problem; that passed all tests. Now that the tools are updated I'm trying going back to path_provider, although this time I'm using path_provider_linux directly to reduce the transitive plugin burden on other platforms. (That was something I had thought about a while back, but slipped my mind when reviewing shared_preferences_linux so wasn't in the initial version.)