Implement new notification LED blinking logic:

1) Do not pulse notification LED when screen is on.
2) Pulse once on new notification if Settings.System.NOTIFICATION_LIGHT_PULSE is false,
   otherwise pulse persistently while screen is off.

Fixes part of bug b/2238250 (trackball should pulse occasionally to indicate new email)

Change-Id: Icc49422a4e9d14412fc159a8e2596503a85bac51
Signed-off-by: Mike Lockwood <lockwood@android.com>