This is to replace the database mutations in the Android SDK. It is
placed behind an `unstable` feature flag until we are satisfied that it
is suitable as a general-purpose API (or replace it).
Per ZIP 316, the Sapling FVK Encoding only includes `(ak, nk, ovk, dk)`
which is a subset of the Sapling `ExtendedFullViewingKey`. We therefore
need to use `DiversifiableFullViewingKey` inside `UnifiedFullViewingKey`
in order to make it parseable from the UFVK string encoding.
`zcash_client_sqlite::wallet::get_extended_full_viewing_keys` has been
removed as a consequence of this change: we can no longer reconstruct
the correct `ExtendedFullViewingKey` from the `UnifiedFullViewingKey`.
This is a breaking change to the database format. We don't have support
for migrations yet, so existing wallets won't work after this commit
until zcash/librustzcash#489 is done.