backend: enabling keystore watchonly should not modify the hidden accounts

The automatically added accounts to be scanned in the background
should not be marked watchonly when the keystore watchonly setting is
enabled. Otherwise, if the accont receives funds, it would appear
automatically even if it was never visible before, even if the
keystore is not connected again.

Author: benma

backend/backend.go

@@ -920,7 +920,7 @@ func (backend *Backend) SetWatchonly(rootFingerprint []byte, watchonly bool) err
 	return backend.AccountSetWatch(
 		func(account *config.Account) bool {
 			// Apply to each currently loaded account.
-			return accounts.lookup(account.Code) != nil
+			return !account.HiddenBecauseUnused && accounts.lookup(account.Code) != nil
 		},
 		&t,
 	)