MMS: The subject disappear after add Sildeshow

If we create a new slideshow. We will drop the old
workingMessage and create a new one. So subject will be
disapper.

Keep the subject from the old mWorkingMessage before we
reassigned.

CRs-Fixed: 409142

Change-Id: I1cc175f8143e702d432d86ae1d69841e8a3a7a47