Convert SignalProxyUtil from Java to Kotlin

[swankjesse]

First time contributor checklist

• I have read how to contribute to this project
• I have signed the Contributor License Agreement

Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Android Emulator with API 36.1

• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

---

Description

I’d like to help migrate Signal from RxJava to MutableStateFlow.

A reasonable place to start is SignalWebSocket. That class has an Observable, state that is used from Java, in SignalProxyUtil. So I’d like to convert that class from Java to Kotlin as a first step. (Java code cannot collect on a MutableStateFlow because it doesn’t support suspend!)

[greyson-signal]

Thank you, much appreciated! The change will go out in 7.67 next week (normally we have weekly releases, but Thanksgiving :)