Convert BitcoinAmountField to use Amount class

Summary: Depends on D747

Test Plan:

1. make check
2. run src/qt/bitcoin-qt --testnet and try sending and receiving some transactions.

Reviewers: deadalnix, freetrader, mbhinder, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D748