Extension: Importing private key should also generate shielded accounts

[jurevans]

We now have the ability to derive shielded keys from private keys in the SDK package.

When a user imports just a private key:

• In addition to the imported transparent address/public key, also derive a spending key and default viewing key/payment address
• Add this to the confirmation UI, as well as View Keys (the shielded key should reference the ID of the transparent SK account as a parent)