[Proof of Concept] Add a persistent mesh network option

[MarkEScheidker]

Description

This is an idea I've been testing that enables bitchat to run in the background with the app closed, while maintaining connection to other users, and still handling PM notifications and new messages. There's also an option to have the network start up automatically after a device restart. Having this as an option will make users more likely to have bitchat running at any given moment, improving mesh size and usability for everyone.

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)
• If it is a core feature, I have added automated tests