simplify ATMP standard flag computation [2/2] - move computation to another function

Summary:
The functional form here is entirely based on GetNextBlockScriptFlags.

(Note: This should entirely remove the need for IsXEnabledForCurrentBlock
functions in future upgrades.)

Depends on D5014

Test Plan: ninja check-all

Reviewers: Mengerian, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5015