Disable the usage summary when there's no usage data.

- when the preference is initialized, check for the network's overall
total usage. If there is no data available, disable launching the detail
page.

Change-Id: Ie81a5471de134bac63dedd3c39f4afe44201efe5
Fixes: 74451774
Fixes: 35855928
Test: make RunSettingsRoboTests