SDK / exoplayer

We have logic to not immediately interrupt playback when an ad group
fails to load and instead let the current content play and transition
at the point where the ad group should have been.

This logic was broken by https://github.com/google/ExoPlayer/commit/dcbdbe53417d6642f2be98c82ac941d34908bd49 because of one of the conditions
used MediaPeriodId.adGroupIndex, which is always -1 for content ids.
It still worked for the last ad group because the next ad group index
was C.INDEX_UNSET.

Fix the issue and amend the test that was meant to catch this to test
the ad failures for the last ad and previous ads.

Also fix the PositionInfo reported in such a case, which was also wrong.

Issue: google/ExoPlayer#9929

#minor-release

PiperOrigin-RevId: 427143223