Refactor: Implement caching and optimize data loading in MainViewModel

This commit introduces a caching layer for app and contact lists in `MainViewModel.kt` to improve performance and responsiveness.

Key changes include:

- **Caching Mechanism:**
    - Implemented in-memory and file-based caching for both app and contact lists.
    - Apps and contacts are now loaded from the cache first for instant display, with background refreshes to update data.
    - Cache files (`apps_cache.json`, `contacts_cache.json`) are stored in the app's internal storage.
    - Added `AtomicBoolean` flags (`appsRefreshing`, `contactsRefreshing`) to prevent concurrent refresh operations.

- **App List Loading Optimization:**
    - `getAppsList` now loads from memory/file cache immediately and triggers a background refresh.
    - Heavy app list processing (querying `LauncherApps`, handling profiles) is moved to `Dispatchers.IO`.
    - User profiles are processed in parallel using `async` to speed up loading.
    - Improved logging for app list generation.
    - Scroll map (`_appScrollMap`) updates are posted to the main thread.
    - Introduced `normalizeForSort` helper function for efficient and consistent sorting of app names.

- **Contact List Loading Optimization:**
    - `getContactList` now loads from memory/file cache immediately and triggers a background refresh.
    - Contact data fetching (names, phones, emails) is moved to `Dispatchers.IO`.
    - Optimized contact querying by first fetching basic contact info and then batch querying phone numbers and emails for all contacts.
    - Scroll map (`_contactScrollMap`) updates are posted to the main thread.
    - Introduced `normalizeForSort` helper function for efficient and consistent sorting of contact names.
    - Added a `ContentObserver` (`contactsObserver`) to monitor changes in `ContactsContract.Contacts.CONTENT_URI`. This invalidates the contact cache and triggers a background refresh when contacts are updated externally.

- **Lifecycle Management:**
    - The `contactsObserver` is registered in `init` and unregistered in `onCleared`.

- **JSON Caching:**
    - Implemented simple JSON serialization/deserialization for app and contact list items using `org.json` for file caching.

Author: CreativeCodeCat