Store ExtFVK with encrypted Sapling spending key instead of FVK

This ensures that even when the wallet is encrypted, we can derive the default
Sapling payment address for our spending keys.

src/wallet/wallet.h

@@ -1060,7 +1060,7 @@ public:
         const libzcash::SaplingExtendedSpendingKey &key,
         const libzcash::SaplingPaymentAddress &defaultAddr);
     bool AddCryptedSaplingSpendingKey(
-        const libzcash::SaplingFullViewingKey &fvk,
+        const libzcash::SaplingExtendedFullViewingKey &extfvk,
         const std::vector<unsigned char> &vchCryptedSecret,
         const libzcash::SaplingPaymentAddress &defaultAddr);