SDK / exoplayer

`ImaAdsLoader` will preload the first ad of a subsequent media item, but the
preloaded ad might not actually play because the user could seek to a non-zero
position in that media item (which could trigger playback of a midroll, not the
preroll). In this case, playback would get stuck because the midroll ad
expected to play after the seek would never load, because the IMA SDK expected
the preroll to play first.

Fix this behavior by discarding the preloaded ad break. If there isn't a seek,
the transition to the next media item is still seamless.

#minor-release

PiperOrigin-RevId: 357682510