fireball: default to MTP USB mode

You can't default to ADB 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.

Change-Id: I90efa1aa8f8c040fe616131cba6939d0a2f4cf57