Preventing more code duplication between whisper_online.py and whisper_online_server.py

Now, the ASR implementations do their own imports. No need to import in the factory

+ refactored creation of the ASR into a factory method

I tried to find the difference between self.last_chunked_at and self.buffer_time_offset, and it took me a while to understand that they are exactly the same. I think it's better to get rid of one of the duplicates to make the code more readable.