Migrate Account type to the new multi-seed-compatible account identifier

[str4d]

Currently the SDK internally uses a ZIP 32 account index, which will be abstracted into an Account type in #1511. However, this will not work with either multiple seeds in the wallet (e.g. both a "daily usage" seed and a Keystone HW seed) or imported viewing keys. Once zcash_client_sqlite has been updated to support a new multi-seed-compatible account identifier, we need to migrate the SDK to use it.