DeskClock: Fix DeskClock crash when click dismiss clock

It is the NullPointerException which cause this crash happen

Add a condition to exclude the NullPointerException.

CRs-fixed: 650830

Change-Id: Id45a1ce757dc1876f56bd5d49ba5376760ec4cb8

(cherry picked from commit 32b783b232d81bbbc29d9ffda1b75ccffe3ed0ef)
DeskClock: Fix crash when click the dismiss in the notification

Startactivity without new task flag will lead app crash. The Deskclock
will start activity from broadcast receiver, this action need add flag
FLAG_ACTIVITY_NEW_TASK

Add the flag of FLAG_ACTIVITY_NEW_TASK for startactivity in onreceive.

CRs-fixed: 690390

Change-Id: I3723a49892ff86b1f54d9ab4b90d8c39f04d0fe5

(cherry picked from commit ac70287f7e8549e7498b1b8efaf4adecb3d3aaa6)