Use the upstreamed StorageStatsSource wrapper.

This was upstreamed into SettingsLib when similar wrappers
started showing up in other projects. Settings, however, was
using the non-upstreamed version.

Change-Id: Iffadb75655165d3b64a25fd7b21e5b9765cb40be
Fixes: 35680800
Test: Settings Robotests still pass despite the data swap out.