NotificationManagerService: Fix up notification LED behaviour
- This should fix the notification LED for Phone and Mms apps (that were
  not using the FLAG_FORCE_LED_SCREEN_OFF introduced by CM cause we use
  our google based ones). Is removed with this one.

maniac103:
- This change also makes the assignment of the LED when multiple pending
  notifications want to use the LED more clear: The notification with
  highest priority gets the LED. If multiple notifications with same
  priority exist, the most recent one gets the LED.