random: simplify FastRandomContext::randbytes using fillrand

Summary:
This is a partial backport of core#28100
https://github.com/bitcoin/bitcoin/pull/28100/commits/44c11769a83b90ca6b8af086d6fa69ff7ac1c3ae

with fillrand() from core#27479 & modifications that should normally be applied in D19034

Depends on D19034

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19035