Add helpers for generating keys, wrapping them, and persisting them

As part of the RecoverableKeyStoreLoader, we need to be able to generate new
256-bit AES keys, sync them with AndroidKeyStore, and persist them, wrapped
to disk. This allows us to recover them later, using a Platform key, and
sync them with remote storage.

Test: manual for now (how do we do automated tests on Framework?)
Change-Id: I32e0beabaecc9bea9f95ca2beea851e9be833358