Unexport the cryptoKey type.
Unexport the cryptoKey type.

The crypto key type unsed in the manager is not needed outside of the
package.  Also, rather than having the newCryptoKey func return the
specific cryptoKey type, make it return the EncryptorDecryptor interface.
This will allow it to be overridden with another type that implements the
interface from the tests.

jrick/btcwallet,decred/dcrwallet,Roasbeef/btcwallet,Stash-Crypto/btcwallet,jrick/dcrwallet,Roasbeef/btcwallet,synrg-labs/btcwallet,btcsuite/btcwallet,jrick/btcwallet,synrg-labs/btcwallet,Stash-Crypto/btcwallet,jrick/dcrwallet,decred/dcrwallet,btcsuite/btcwallet