[backport#16822 3/3] gui: fix autofocus in CreateWalletActivity::askPassphrase()

Summary:
Backport of Core PR16822 - part 3 of 3
Commit https://github.com/bitcoin/bitcoin/pull/16822/commits/cad3ab5db835e1d0c44c6a5fc0e4b2d1661dcd5c

Depends on D8007

Test Plan: Run src/qt/bitcoin-qt, create a new wallet with encryption and check that the dialog is modal.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8008