Fix ad tag loader period index

Previously it was safe to query the first period in the timeline, but
now we support using the ads loader in concatenations we need to use
the current period index instead.

#minor-release

PiperOrigin-RevId: 357578003

RELEASENOTES.md

@@ -20,6 +20,10 @@
         instead.
     *   Remove `extension-jobdispatcher` module. Use the `extension-workmanager`
         module instead.
+*   IMA extension:
+    *   Fix a bug with playback of ads in playlists, where the incorrect period
+        index was used when deciding whether to trigger playback of an ad after
+        a seek.
 *   VP9 extension: Update to use NDK r22
     ([#8581](https://github.com/google/ExoPlayer/issues/8581)).
 *   FLAC extension: Update to use NDK r22

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/AdTagLoader.java

@@ -826,7 +826,7 @@ import java.util.Map;
       ensureSentContentCompleteIfAtEndOfStream();
       if (!sentContentComplete && !timeline.isEmpty()) {
         long positionMs = getContentPeriodPositionMs(player, timeline, period);
-        timeline.getPeriod(/* periodIndex= */ 0, period);
+        timeline.getPeriod(player.getCurrentPeriodIndex(), period);
         int newAdGroupIndex = period.getAdGroupIndexForPositionUs(C.msToUs(positionMs));
         if (newAdGroupIndex != C.INDEX_UNSET) {
           sentPendingContentPositionMs = false;