Fix crash when the live default position moves past a playing SSAI ad

[oguzhaneksi]

Fixes #3348.

Problem

Joining a live SSAI stream (e.g. AWS MediaTailor) while an ad break is on air, and
then receiving a manifest refresh that moves the window default position past that
ad, crashes the playback thread:

java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
  at androidx.media3.common.AdPlaybackState.getAdGroup
  at androidx.media3.common.Timeline$Period.isServerSideInsertedAdGroup
  at androidx.media3.exoplayer.ExoPlayerImplInternal.isIgnorableServerSideAdInsertionPeriodChange
  at androidx.media3.exoplayer.ExoPlayerImplInternal.resolvePositionForPlaylistChange
  at androidx.media3.exoplayer.ExoPlayerImplInternal.handleMediaSourceListInfoRefreshed

Root cause

resolvePositionForPlaylistChange passes the period looked up by newPeriodUid to
isIgnorableServerSideAdInsertionPeriodChange, assuming it is the period that
periodIdWithAds refers to. That assumption doesn't hold:
MediaPeriodQueue.resolveMediaPeriodIdForAdsAfterPeriodPositionChange rolls back to a
preceding unplayed server-side inserted ad period, so the returned MediaPeriodId can
belong to a different period than newPeriodUid.

For a live window |- p0 content -|- p1 ad -|- p2 content -| where playback joined
inside the ad in p1 and the refreshed default position lands in p2:

• newPeriodUid → p2, which has an empty AdPlaybackState
• periodIdWithAds → p1, adGroupIndex = 0

The uid equality guard inside isIgnorableServerSideAdInsertionPeriodChange compares
oldPeriodId.periodUid with newPeriodId.periodUid — both p1, so it passes — and the
following line then indexes p2's empty AdPlaybackState with p1's ad group index,
throwing ArrayIndexOutOfBoundsException.

This only reproduces when the refreshed default position lands in a period after the
ad. If it lands before the ad (p0), the uid guard returns early and the faulty line is
never reached, which is why the crash looks position-dependent.

Fix

Look the period up by the resolved period uid (periodIdWithAds.periodUid) instead of
newPeriodUid. Past the uid guard, oldPeriodId and newPeriodId share a period uid
by construction, so the resolved uid is the only correct one to use here.

With the fix, the change is correctly detected as an in-stream ad change, the update is
dropped and the ad keeps playing across the refresh — no renderer reset.

Testing

Added ExoPlayerAdTest.timelineRefresh_movingLiveDefaultPositionPastPlayingSsaiAd_keepsPlayingAd,
which fails with the ArrayIndexOutOfBoundsException above without the fix and passes
with it.

Ran on :lib-exoplayer: ExoPlayerAdTest (64), ExoPlayerTest (2180),
MediaPeriodQueueTest (58), AdsMediaSourceTest (27),
ServerSideAdInsertionMediaSourceTest (32), ServerSideAdInsertionUtilTest (8) —
2369 tests, 0 failures.

[tonihei]

Thanks for the investigation and providing a fix! It looks correct to me and I'll start the internal merge process.