Pyramid: default to MTP

We can't use adb by default since user is required to enable USB debugging
before adb kicks in. UMS was deprecated to help save users from
themselves (unclean unmounts and apps left wondering what happened to
their storage volume). We should at least default to MTP, even if users
will opt to change this later.
base on this: http://review.cyanogenmod.org/#/c/63213/