Should accept "application/vnd.wap.multipart.alternative" message.

Unfortunately, PduParser does not allow application/vnd.wap.multipart.alternative as M-RETRIEVE.CONF but some Mobile carrier send message with this type.
This patch allows that and take the first part of multipart as message body.

It fixes http://code.google.com/p/android/issues/detail?id=8957

Change-Id: Ic93259c91331d1e67100439114b7c4f43a834368