refactor: Get rid of Wallet::IsWalletFlagSet method

Summary:
Replace by privateKeysDisabled method to avoid need for GUI to reference
internal wallet flags.

Also remove adjacent WalletModel canGetAddresses wrapper that serves no purpose
and make Wallet::canGetAddresses non-const so it can be implemented by IPC
classes in #10102.

Backport of Core PR18278 part [1/6] : https://github.com/bitcoin/bitcoin/pull/18278/commits/77e4b0657298c715c835d8d2eb11e173852e6815

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8019