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

@@ -65,9 +65,11 @@ 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()
+    final SharedPreferencesAsync sharedPreferences = SharedPreferencesAsync();
-        .then(
+
-          (sharedPreferences) => sharedPreferences.setBool('isDebug', true),
+    final Future<void> sharedPreferencesSet = sharedPreferences.setBool(
+      'isDebug',
+      true,
     );
     final Future<Directory> tempDirFuture = path_provider
         .getTemporaryDirectory();

compass_app/app/integration_test/app_server_data_test.dart

@@ -30,7 +30,7 @@ void main() {
 
     setUpAll(() async {
       // Clear any stored shared preferences
-      final sharedPreferences = await SharedPreferences.getInstance();
+      final sharedPreferences = SharedPreferencesAsync();
       await sharedPreferences.clear();
 
       // Start the dart server

compass_app/app/lib/data/services/shared_preferences_service.dart

@@ -13,9 +13,11 @@ class SharedPreferencesService {
 
   Future<Result<String?>> fetchToken() async {
     try {
-      final sharedPreferences = await SharedPreferences.getInstance();
+      final sharedPreferences = SharedPreferencesAsync();
+
       _log.finer('Got token from SharedPreferences');
-      return Result.ok(sharedPreferences.getString(_tokenKey));
+
+      return Result.ok(await sharedPreferences.getString(_tokenKey));
     } on Exception catch (e) {
       _log.warning('Failed to get token', e);
       return Result.error(e);
@@ -24,14 +26,18 @@ class SharedPreferencesService {
 
   Future<Result<void>> saveToken(String? token) async {
     try {
-      final sharedPreferences = await SharedPreferences.getInstance();
+      final sharedPreferences = SharedPreferencesAsync();
+
       if (token == null) {
         _log.finer('Removed token');
+
         await sharedPreferences.remove(_tokenKey);
       } else {
         _log.finer('Replaced token');
+
         await sharedPreferences.setString(_tokenKey, token);
       }
+
       return const Result.ok(null);
     } on Exception catch (e) {
       _log.warning('Failed to set token', e);