Fix a bug where delete alarm toast sometimes doesn't show.

Bug: 22932712

There is a race condition The show toast logic is in a DataSetObserver
and it checks mDeletedAlarm must be present. Sometimes loader's
onLoadFinish() happens before DataSetObserver's onChanged(), and it
wipes mDeletedAlarm, so DataSetObserver doesn't have a chance to show
the toast.

I moved show toast logic to onPostExecute section of AsycnDeleteAlarm.
This is the same mechanism for displaying the toast for adding alarm.

Change-Id: I452b03ea26e22f0638c1223a0a0c64c2ec82707d