Mms: Fix can't cancel downloading MMS when tap "OK"in confirm box.

Download state is changed to un-started after canceled MMS, and MMS will
show "Cancel" button both the state of un-started and downloading.

Change download state to transient failure if cancel download.

CRs-Fixed: 743500

Change-Id: I3b9789714b3e7bd4b0f17816bca7fe3f0f1468de