Begin
Marks the start of a presentation. A presentation is identified by its paywall_open: if the current id has already been reported by one (see SetLastOpen), or the view never got one at fetch time, a new id is minted here. Otherwise the id is kept, so a fresh view's load events and its first open share the id minted at fetch time.
Idempotent between opens, so it can run both when a presentation is being prepared (PaywallView.beforeViewCreated) and when the view is created (PaywallView.onViewCreated). Everything tracked between two opens - paywall_close, transaction_* - keeps the id of the open that preceded it, even if the view was re-bound in between.