If we use the default start position, we currently resolve it immediately
even if we need to play an ad first, and later try to project forward again
if we believe that the default start position should be used.
This causes problems if a specific start position is set and the later
projection after the preroll ad shouldn't take place.
The problem is solved by keeping the content position as TIME_UNSET (= default
position) if an ad needs to be played first. The content after the ad can
then be resolved to its current default position if needed.
PiperOrigin-RevId: 258583948
049f3cf5cd