Migrate compass_app to SharedPreferencesAsync

[harshyadavDeveloper]

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

• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I read the [Contributors Guide].
• I have added sample code updates to the [changelog].
• I updated/added relevant documentation (doc comments with ///).

[harshyadavDeveloper]

Implemented the migration from SharedPreferences to SharedPreferencesAsync in compass_app as discussed in #2720.

The changes include:

• replacing legacy SharedPreferences.getInstance() usage
• updating async getters
• migrating token persistence logic
• updating related test setup code

Feedback and review are welcome. Thanks!