overlay: mms: increase mms image size

Change-Id: Ifd3767806144f1aa0d261851c2078dc0cc16fe2f
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