Add support for default data subscription setting.

Default data subscription setting holds the default preference for data
subscription. It will be updated when user changes data preference,
sub deactivated or card removal.

Current set preferred data subscription API does not update the default
data subscription preference. This change is to handle temporary DDS switch
by apps like MMS.

New APIs to update and retrieve the default data subscription are added.

Change-Id: I495266cff2a5319c57279d9decb77d6a87bbfae8
CRs-Fixed: 642210