overlay: mms: add useragent

* apparently this is required in this overlay, i guess it would make
  too much sense for it to use the default when not overlayed like
  every other overlay...

Change-Id: Icdfd5e73070d62d42cd4fafbc2e18757453647fa
(cherry picked from commit 00ad35eb36a219a83ec2dfbf13a3690665bb5823)