By default, Android exposes USB devices only to the main user.
In order to query, read and write to it, the signature permission INTERACT_ACROSS_USERS_FULL (optional) is granted to create Seedvault's context as the system user.
Issue: calyxos#437
Issue: https://github.com/seedvault-app/seedvault/issues/77
Change-Id: I0b1b4c8c5aeeb226419ff94e15f631ebe1db66df