DeskClock: Set FLAG_ACTIVITY_NEW_TASK explicitly

* From PendingIntent#getActivity() documentation:
  "Note that the activity will be started outside of the context of
  an existing activity, so you must use the
  Intent.FLAG_ACTIVITY_NEW_TASK launch flag in the Intent."
* Removes the warning
  ActivityTaskManager: startActivity called from non-Activity context;
  forcing Intent.FLAG_ACTIVITY_NEW_TASK for: Intent {flg=0x800000
  cmp=com.android.deskclock/.timer.* }

Change-Id: I628a5b7fa9e8b00adca3d384dc0a12cb07b8efa8