Mms: disable SplitActionBar

This makes the UI more consistent with newer Email app look.

Screenshots:
  https://dl.dropboxusercontent.com/u/16268774/cm_temp/email.jpg
  https://dl.dropboxusercontent.com/u/16268774/cm_temp/mms.jpg

Change-Id: I71c287f258b9cb8f4b94456ecc9d4034d4680525