This makes sure the number of ads in an ad group matches to the number of periods representing an ad group in a multi-period timeline. This makes it easier to accurately mark ads as played in multi-period windows which is needed to correctly prevent seeking over unplayed ads.
PiperOrigin-RevId: 425317085
Where this introduced an inconsistency (e.g. assigning to something
called `windowIndex`), I generally renamed the transitive closure of
identifiers to maintain consistency (meaning this change is quite
large). The exception is code that interacts with Timeline and Window
directly, where sometimes I kept the 'window' nomenclature.
#minor-release
PiperOrigin-RevId: 407040052
