Mms: Add the retrieve conf type support in MMS PDU composer.

The sent MMS PDU type is MESSAGE_TYPE_RETRIEVE_CONF, PduComposer does
not support it and always return null, so it fails when backup Mms.

Add this type check in PduComposer accordingly.

Change-Id: I57444cd8a9ddc2db4586088a05600f21331ef465