use sqlite to store paired auditee data

[master-hax]

the app currently uses SharedPreferences to store auditee pairings, with one file for one paired auditee.

moving the paired auditee data to a sqlite database makes a lot of sense:

1. it becomes possible to enumerate all the devices in a consistent ordering without having to do sorting
2. the web app already uses sqlite, so this can help unify the two apps.
3. using many SharedPreferences files significantly increases memory usage as iiuc each SharedPreference file gets loaded & cached in memory
4. the graphene os development guidelines explicit call out sqlite as a trusted library