Migrate compass_app to SharedPreferencesAsync (#2841)

## Description

Migrated `SharedPreferences` usage in `compass_app` to
`SharedPreferencesAsync` to align with the latest `shared_preferences`
recommendations.

### Changes made

* Replaced `SharedPreferences.getInstance()` usages
* Updated async preference reads with awaited getters
* Migrated token persistence logic
* Updated test setup preference clearing logic

Fixes #2720

## Pre-launch Checklist

* [x] I read the [[Flutter Style
Guide](https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md)]
*recently*, and have followed its advice.
* [x] I signed the [[CLA](https://cla.developers.google.com/)].
* [x] I read the [[Contributors
Guide](https://github.com/flutter/samples/blob/main/CONTRIBUTING.md)].
* [ ] I have added sample code updates to the
[[changelog](https://chatgpt.com/CHANGELOG.md)].
* [ ] I updated/added relevant documentation (doc comments with `///`).

<!-- Links -->

[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md
[changelog]: ../CHANGELOG.md

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

background_isolate_channels/lib/main.dart

@@ -65,10 +65,12 @@ class _MyHomePageState extends State<MyHomePage> {
     // Write the value to [SharedPreferences] which will get read on the
     // [SimpleDatabase]'s isolate. For this example the value is always true
     // just for demonstration purposes.
-    final Future<void> sharedPreferencesSet = SharedPreferences.getInstance()
-        .then(
-          (sharedPreferences) => sharedPreferences.setBool('isDebug', true),
-        );
+    final SharedPreferencesAsync sharedPreferences = SharedPreferencesAsync();
+
+    final Future<void> sharedPreferencesSet = sharedPreferences.setBool(
+      'isDebug',
+      true,
+    );
     final Future<Directory> tempDirFuture = path_provider
         .getTemporaryDirectory();