Bug 919962 - [MMS] Converting to text layout when sending a MMS r=gnarf

We now display the banner in an asynchronous way therefore the previous way to
hide the banner synchronously didn't work.

This patch changes the logic then: instead of hiding the banner after displaying
it, it now prevents the banner from being displayed at all.
Merge pull request #12514 from julienw/919962-dont-show-sms-conversion-banner-after-send

Bug 919962 - [MMS] Converting to text layout when sending a MMS r=gnarf