fix: never create a new asyncio loop

[allenporter]

Always reuse the existing asyncio loop rather than creating a new one.

This is needed to upgrade python-roborock in Home Assistant since the async shutdown call must happen in the same asyncio loop that was used to create the client.

This requires inlining all calls to get the currently running loop, because the constructor for the MQTT client needs to be created from outside the asyncio loop because the MQTT client does blocking I/O in the constructor. (example: home-assistant/core#124266). In the future we can move this back to the constructor, but that requires separately creating the MQTT client in a way that doesn't block, which we can do in the future when using a shared MQTT client as proposed by @Lash-L.