BACKUP: Fix Phone tip "backup has stopped" when backup mms to SD card .

The send out mms's pdu type is Retrieve Conf, but the PduComposer didn't
support this type, so always return null, this will cause backup failed.

Add Retrieve Conf support for PduComposer.

CRs-Fixed: 731522

Change-Id: I57444cd8a9ddc2db4586088a05600f21331ef465