New login flow should not create 2 sessions

The new login flow used by the clients creates two sessions.

1. The session when logging in
2. The session for the apptoken that is created

The first session should not be created (or just killed directly once the apptoken is generated). This clutters the sessions and makes it look weird and uncertain which one to delete.

Ref: https://github.com/nextcloud/android/issues/1436