refactor: replace direct usage of K9.contactNameColor with PreferenceDataStore

[shamim-emon]

• Fixes Replace Direct Usage of K9.contactNameColor With PreferenceDataStore #10242

Summary

This PR refactors the way the contact name color is handled in Thunderbird Android. Instead of using the direct reference K9.contactNameColor, it now leverages PreferenceDataStore to manage this setting consistently across the app.

Changes Made

• Removed direct usage of K9.contactNameColor.
• Updated tests (MessageHelperTest and related files) to reflect the change in preference management.
• Minor cleanup to improve maintainability and readability of code that interacts with contact name colors.

Motivation

Using a centralized preference store instead of direct variable references ensures:

• Consistency: All reads and writes to contact name color go through a single source of truth.
• Maintainability: Future changes to how contact colors are stored or retrieved will require minimal code modifications.
• Extensibility: Prepares the codebase for features like theme-specific or account-specific contact color settings.

Impact on Users
This change is internal and does not affect the app’s behavior from a user perspective. Users will continue to see contact name colors as before.

Testing

• Verified existing tests pass after refactoring.

[wmontwe]

LGTM