Code refactor+migrate to data store

[xanderlmk]

Description

• Code refactor for BluetoothMeshService and ChatViewModel.
• Update BluetoothMeshDelegate and create an implementation which overrides BluetoothMeshDelegate rather than just the ChatVM.
• Updates to MainActivity
• Change Theme saving to data store instead of shared preferences
• Replace Google Gson to native kotlinx Json
• Add a package directory for application

Why It matters

• As code grows, it's best to separate components for readability and maintainability.
• DataStore allows for automatic flows without the need to have two separate variables compared to just SharedPreferences.
• Kotlin's native kotlinx.serialization library.Serialization is much easier to use and manipulate and is better performing than Google's Gson

TODO

• Finish migration from SharedPreferences to DataStore
• Completely remove GSON and use JSON

Suggestions

• Room is a framework that allows pretty good persistence storage and allows for storing for more complex objects and classes. It is just a wrapper for SQLite.
• More code refactor for better readability.

Checklist

• I have read the contribution guidelines: https://github.com/permissionlesstech/bitchat-android?tab=readme-ov-file#contributing
• I have performed a self-review of my code

• I have mentioned the corresponding issue and the relevant keyword (e.g., "Closes: #xy") in the description (see https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
• Make sure application works as intended using testing devices

[shubertm]

It may be good if the name was meaningful, like meshDelegate or full bluetoothMeshDelegate

[shubertm]

Better to have data migrations considering that the app is already distributed to users. And could also help removing usage of SharedPreferences completely