Mms: Always display "building slideshow" when reject MT call

The "Building slideshow" dialog dismiss before onPause(),
onPause() method will be called in advance and the value of
dialog will be set to null when reject a incoming call after
click "Edit" button.The dialog has no chance to dismiss.

If onPause() method is called in advance,dismiss the dialog
and set the value of dialog to null.

CRs-Fixed: 427787

Change-Id: I8db748e70e5f6420500ce6da161f2f6c2593af0b