core: use 'normal' value for notification led off

Users can set this value to 250, 500, 1000, 2500 or 5000ms only,
but it's 2000ms by default.

With this patch, 'Normal' string is shown instead of 'By default'
and pulse speed goes to 1 sec.

Change-Id: Idb18ac345bd9eb1a5b795776faa60e2fb5bc258b
Merge "core: use 'normal' value for notification led off" into du44