settings: notification transparency (2/2)

this add back notification transparency...compared to 4.3
it controlls as well the notification icon backgound transparency
which felt in the past dissplaced. As well codecleanup of the old
implementation.

largeIcon support I did not add due that google does not support it
as well at current status...it is only a placeholder in
NotificationData

Change-Id: Iacefd9d02919990517823e5d800c6e018c3c68a1

Conflicts:
	res/values/slim_strings.xml