Persist "mask values" to prevent shoulder surfing

[fanquake]

Current Behavior: "Mask values" mode is off each launch and must be manually re-enabled. If a wallet loads on startup from settings.json this leaves user vulnerable to shoulder surfing.

Solution: This mode should persist across restarts of bitcoin-qt by being saved in Bitcoin-Qt.conf. Exactly how nDisplayUnit=0 will persist the unit to show amounts in.

Expected Behavior: If I shutdown Bitcoin core with it on, I'd expect it to be on when I restart and vice versa.

Moved from: https://github.com/bitcoin/bitcoin/issues/25826.