Bug 1061215 - [Messages][Refactoring] Improve handling of remaining chars counter. r=schung

Replied to that too :) 
> Yes, we can and should, I'm doing it in the next patch for the top part, and reduce ids for the bottom part in the next one (making Compose standalone component) :) If you don't mind :)

I can do it in this patch or in the next as I planned. What do you prefer?