Fix PDU part ordering in PduParser

Nova does not send SMIL. When sending multiple images, the second image
part and so on would match content-type "type" param and being inserted
in reverse order (PduParser.checkPartPosition). This change makes sure
if "start" param does not match part's content id, the part will be
appended to the list and shortcut the "type" check.

b/19607294

Change-Id: Idfebd0cac35b637e4d6d6513d5dc6170d7262ff1