DeskClock: handle call events on alarm

Alarm clock ringing while incoming call received.

When call ringing,alarm clock time arrived.Only Call ringing and no alarm
clock ringing.and after hang up the phone,alarm start ring.
When alarm ringing,receive a call,the alarm is stopped,and it can answer
or hang up the phone.after hang up the phone ,there has some reminder.

Change-Id: Id1bdf689f709e78ced30cec06cbc93a7a06a5b3b
CRs-Fixed: 638868