[electrum] move .encrypt method to ECPubkey class

Summary:
This just makes more sense than having it as a classmethod of the private key class and requiring the pubkey to be provided as an argument.

Depends on D16675

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16677