wallet: Refactor to WalletInitInterface* const g_wallet_init_interface

Summary:
Backport 2/2 of core PR12836 (commit d894894)

Depends on D2273

Test Plan:

make check

Check that bitcoind and bitcoin-qt can be executed

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2274