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