[explorer] Determine the tx finalization status from the dedicated field

Summary:
Instead of looking at the containing block finalization status we use the tx.is_final field directly.

Depends on D18490.

Test Plan: Run the preview and check it works.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18491