Query correct subscription id for mobile data.

- On the Data usage settings page, we check for valid subscription id to
determine if we should show the mobile section. However, we are querying
the default subscription id, which will return the 1st id for any
subscription if there's no data subscription. Since this is for mobile
section, we should query the default data subscription id explicitly.

Change-Id: I08c80639d07477e486a2d9a10761e11df4c73b28
Fixes: 119881046
Test: make RunSettingsRoboTests