[refactor] get ValueFromAmount via an implicit Amount::operator UniValue()

Summary:
the number of decimals of precision that an Amount represents is currently all over the place on the codebase, this is one step to getting it all under one roof.

this seems more correct as well, representing as a UniValue is the Amount's job, not some free function. also less verbose code.

AmountFromValue probably should be an Amount constructor next.

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9603