Use UnifiedSpendingKey in AccountUsk rather than its encoding

[daira]

Originally posted by @nuttycom in #1640 (comment) :

/**
 * The binary encoding of the [ZIP 316](https://zips.z.cash/zip-0316) Unified Spending
 * Key for [accountUuid].
 *
 * This encoding **MUST NOT** be exposed to users. It is an internal encoding that is
 * inherently unstable, and only intended to be passed between the SDK and the storage
 * backend. Wallets **MUST NOT** allow this encoding to be exported or imported.
 */
private val bytes: FirstClassByteArray

Why is this not UnifiedSpendingKey?