Lockscreen Notification: default values and improve onSettingsChanged

Add default values that are used in the frameworks/base part.

Check for the key instead of the table in onSettingsChanged.

Change-Id: Ia90c3d3eb4d55933d04f9a80926de7b192259526
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>