SDK / exoplayer

On receiving a fetch error for an ad that would otherwise play based on an
initial/seek position, the pending content position wasn't cleared which meant
that position reporting was broken after a fetch error. Fix this by always
clearing the pending position (if there was a pending position that will have
triggered the fetch error).

Also deduplicate the code for handling empty ad groups (fetch errors)
and ad group load errors.

Issue: #7956
PiperOrigin-RevId: 334113131