SystemUI: Don't pad when config_showScreenOnLockScreenHints is false

When the overlay is false the 'down' indicator is unused and there-
fore no padding for text alignment is needed.

Also use the overlay to disable the up-arrow indicator, thus return-
ing to the old lockscreen layout.

RM-290

Change-Id: I542109686778e081c1883885287de0d5380578a2