secret: Always use the schema attribute to get secrets

Don't rely on whether the app is sandboxed to guess the keyring backend. Avoids weird behavior when the app is sandboxed but oo7 falls back to the dbus service (because the user gave them access).

Migrate secrets in the file (portal) backend to include the schema attribute, that way we can use a single way to get the items. Note that this migration can be removed when we release the stable version.

Closes Nightly removes secret entries that don’t belon... (#1113 - closed).
Closes Look for an item with all required attributes w... (#1114 - closed).