Bug 947139 - [DSDS][Message] Send message via default SIM r=schung
Follow-ups:

* The container height needs to take the form's padding into account, otherwise
  we loose the bottom padding in the message list
* Add documentation for the new functions
* Made clearer that some functions are internal
* renamed the local `doSendMessage` function to `next`
* extracted in its own function the code that builds a `sendOpts` object that we
  use in both sendSMS and sendMMS.