[backport#16106] refactor: Rename getWallets to getOpenWallets in WalletController

Summary:
https://github.com/bitcoin/bitcoin/pull/16106/commits/fa90fe6440301edba71a6f0b1ba3ef1c78d5eae4

Depends on D6174

Completes backport of Core PR16106

Test Plan:

ninja

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6175