Fix up notification LED behaviour

- Commit 5713307e3d2eb457ba148df3683ad09733727506 (in ICS) attempted to
  fix the problem that the notification LED wasn't reenabled after
  turning off the screen again, but introduced another bug in the
  process: If another pending notification with LED at the time when the
  notification with the new magic flag introduced by that commit (which
  was used by phone and MMS apps) was active, the LED still wouldn't be
  turned on at screen off time. Also, this commit didn't handle locking
  correctly.
  This commit essentially reverts 5713307e and makes sure the LED is
  reevaluated when the screen is turned off again.

- 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.