The assertion asserts against a `Period` and an `AdPlaybackState` which actually
asserts against a resolved ad which is what `ExoPlayerImplInternal` does later and
what gives us a `SEEK_ADJUSTMENT`. However, this assertion is not required at the
moment of masking, because we are sure that the resolved seek results in a content
period and never an ad period.
#minor-release
Issue: androidx/media#122
PiperOrigin-RevId: 471827072
