wallet: Make WalletInitInterface members const

Summary: This is partial backport of Core PR12977 : https://github.com/bitcoin/bitcoin/pull/12977/commits/1936125671fbdb3411fcdf0d09fbd7ced32272e6

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3045