[url_launcher] Improve README and example (flutter#4529)

Fixes the README and example to properly demonstrate using `canLaunch`
to adjust UI, rather than as a direct wrapper around `launch` (which
serves no useful purpose given that `launch` can already return
failure).

Also makes some minor organizational improvements to the README (e.g.,
fixing the fact that the discussions of iOS and Android configuration
were separated by the inline example for some reason).

Enable Android integration tests in remaining plugins (flutter#4514)

Enable Android integration tests in remaining plugins (flutter#4514)

[shared_preferences] Publish fully federated version (flutter#4526)

Restores the app-facing package to publishable state, pointing at the
newly-published federated Android and iOS implementation packages.

Part of Part of flutter/flutter#68498

[shared_preferences] Federate mobile implementations (flutter#4505)

Fully federates the plugin by moving the existing mobile implementations to their own packages, per planned repo structure.

Temporarily marks shared_preferences as unpublishable to allow the implementations to be moved, rather than copied and deleted, in order to better preserve git history. A follow-up PR will restore it to publishable form.

Part of flutter/flutter#68498

[shared_preferences] Federate mobile implementations (flutter#4505)

Fully federates the plugin by moving the existing mobile implementations to their own packages, per planned repo structure.

Temporarily marks shared_preferences as unpublishable to allow the implementations to be moved, rather than copied and deleted, in order to better preserve git history. A follow-up PR will restore it to publishable form.

Part of flutter/flutter#68498

Bump plugin Android compileSdkVersions to 31 (flutter#4502)

Bump plugin Android compileSdkVersions to 31 (flutter#4502)

[in_app_purchase] Emit empty list when no transactions to restore. (f…

…lutter#4523)

[in_app_purchase] Rename iOS implementation to _storekit (flutter#4518)

Renames the iOS implementation package to `in_app_purchase_storekit`, and renames it types and files accordingly, to allow for adding a symlink-based macOS implementation to this package in the future.

Temporarily marks `in_app_purchase` as unpublishable so that this can be a move, rather than a copy-and-delete-later, which makes for difficult reviews and destroys git blame.

Part of flutter/flutter#86076
Supports flutter/flutter#84391