Don't generate an auth picker view singleton 

in the sample app, auth picker view should be regenerated each time "sign in" is tapped. Otherwise the old view won't catch up with the most recent idp list changes. 
the bug was introduced in this commit https://github.com/firebase/FirebaseUI-iOS/commit/9ac4880fdbe47079759e26fe956f1c8900fc81ad#diff-8ab9ec2ba462ce4f45b98fff5f5f34bc 