Update some plugins targetSdkVersions to 31 (flutter#4535)

Precursor to flutter#4533

On master channel, plugin unit tests must now build with SDK version 31.
These plugins are transitive dependencies of other plugins' examples,
which causes unit test failures in those plugins's unit test builds, so
these need to be landed and publish in order for that PR to pass
presubmit.

Update some plugins targetSdkVersions to 31 (flutter#4535)

Precursor to flutter#4533

On master channel, plugin unit tests must now build with SDK version 31.
These plugins are transitive dependencies of other plugins' examples,
which causes unit test failures in those plugins's unit test builds, so
these need to be landed and publish in order for that PR to pass
presubmit.

Update some plugins targetSdkVersions to 31 (flutter#4535)

Precursor to flutter#4533

On master channel, plugin unit tests must now build with SDK version 31.
These plugins are transitive dependencies of other plugins' examples,
which causes unit test failures in those plugins's unit test builds, so
these need to be landed and publish in order for that PR to pass
presubmit.

[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)