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.
Small cleanup
Give floating flag to previous activities in the stack
When launching stacked activities, only the last one is flagged as floating in sysui when tapping halo.
This causes the system to show the launcher when the first stacked activity is launched (this first intent gets the flag FLAG_ACTIVITY_TASK_ON_HOME
in TaskStackBuilder).
This change makes all the stacked activities get the floating flag if the last one has it.