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

Thanks! About ```currentSegment > 0 && this.previousSegment > 0``` I think about the case when user copies some text that occupies several segments and paste it to the message input (I believe it will possible in v2.2 since copy & paste will be supported) - so we probably need to show SMS segment banner in this case too (```currentSegement === 2/3/4 and previousSegment === 0```).