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

I'd rather add here one more check like ```!(this.previousSegment === 0 && currentSegment === 1)``` to not show notice for the first entered char to work as in master. But at the same time I see that in master we show message like "Currently editing SMS 0" when I remove all chars. So asking @steveck-chung how it should work before I make a change here :)

* for the first entered char;
* when we remove all chars;
* when we go from segment 1 to 2;
* when we go from segment 2 to 1;