wallet: Refactor g_wallet_init_interface to const reference

Summary:
This is the second part of Core PR12977 : https://github.com/bitcoin/bitcoin/pull/12977/commits/6ec78f146134be7ee067a25a412d3c927f9ac353

Depends on D3045 and D3047

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3048