Default time format is hard coded to 24

The items "def_time/date_format" will be hard coded for Default.
So should set "def_time/date_format" to be null in order to bring
back the AOSP implementation of picking the date format based on
the locale for Default

OPO-431
Change-Id: Ia5d74662782bcc00a8173fd49604f187a116468e