Bug 902497 - [sms][mms] use the new asynchronous getSegmentInfoText from...

... bug 903403 for updateSmsSegmentLimit r=schung

* we don't call getSegmentInfoForText more than once every 500ms
* getSegmentInfoForText is now asynchronous and the new code is handling this
* we now do the composeForm dataset type change in ThreadUI. As a result, the
  "type" event, sent by Compose, is not cancelable anymore, because we don't use
  this anymore.