fix(SalesforceSDKCore): gate host list back button on forced-advanced-auth flag

shouldShowBackButton returned YES for any host list VC whenever the
config's shouldDisplayBackButton was true, without checking
showsBackButtonAndLoginOptions. shouldDisplayBackButton is
!appConfig.shouldAuthenticate, which is YES in environments without a
bootconfig, so the back button leaked onto instances outside the
forced-advanced-auth path.

Add the showsBackButtonAndLoginOptions guard already used on the sole
production caller (line 159) and the login-options gear (line 275) so
the method is self-consistent. Fixes the failing unit test
test_givenUnlockedNoFlowNoAccount_whenShouldShowBackButton_thenNo.

Author: brandonpage

libs/SalesforceSDKCore/SalesforceSDKCore/Classes/Login/LoginHost/SFSDKLoginHostListViewController.m

@@ -226,7 +226,8 @@ - (BOOL)shouldShowBackButton {
     }
 
     SFUserAccountManager *accountManager = [SFUserAccountManager sharedInstance];
-    if ([self.config isKindOfClass:[SFSDKLoginViewControllerConfig class]]
+    if (self.showsBackButtonAndLoginOptions
+        && [self.config isKindOfClass:[SFSDKLoginViewControllerConfig class]]
         && ((SFSDKLoginViewControllerConfig *)self.config).shouldDisplayBackButton) {
         return YES;
     }