Let UpdateTime take the adjusted time instead of getting it locally

Summary: For the heartbeat feature we want to return the current node time in the block template. In order to avoid time point discrepeancy, the time should be accessed once and passed down the places where it's needed. There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16594