Work with PkDecryption keys by their private keys

Change interface to allow the app to get the private part of the
key and instantiate a decryption object from just the private part
of the key.

Changes the function generating a key from random bytes to be
initialising a key with a private key (because it's exactly the
same thing). Exports & imports private key parts as ArrayBuffer at
JS level rather than base64 assuming we are moving that way in
general.