Fix resuming postrolls

Postrolls would be skipped because the period duration wasn't know at
the moment of resuming playback after backgrounding, so the position
wouldn't be resolved to resume the postroll ad.

We have the period duration stored in the AdPlaybackState, so we can
use that directly.

Issue: #7518
PiperOrigin-RevId: 317830418

library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdPlaybackState.java

@@ -270,7 +270,9 @@ public final class AdPlaybackState {
   public final AdGroup[] adGroups;
   /** The position offset in the first unplayed ad at which to begin playback, in microseconds. */
   public final long adResumePositionUs;
-  /** The content duration in microseconds, if known. {@link C#TIME_UNSET} otherwise. */
+  /**
+   * The duration of the content period in microseconds, if known. {@link C#TIME_UNSET} otherwise.
+   */
   public final long contentDurationUs;
 
   /**

library/core/src/main/java/com/google/android/exoplayer2/source/ads/SinglePeriodAdTimeline.java

@@ -44,23 +44,16 @@ public final class SinglePeriodAdTimeline extends ForwardingTimeline {
   @Override
   public Period getPeriod(int periodIndex, Period period, boolean setIds) {
     timeline.getPeriod(periodIndex, period, setIds);
+    long durationUs =
+        period.durationUs == C.TIME_UNSET ? adPlaybackState.contentDurationUs : period.durationUs;
     period.set(
         period.id,
         period.uid,
         period.windowIndex,
-        period.durationUs,
+        durationUs,
         period.getPositionInWindowUs(),
         adPlaybackState);
     return period;
   }
 
-  @Override
-  public Window getWindow(int windowIndex, Window window, long defaultPositionProjectionUs) {
-    window = super.getWindow(windowIndex, window, defaultPositionProjectionUs);
-    if (window.durationUs == C.TIME_UNSET) {
-      window.durationUs = adPlaybackState.contentDurationUs;
-    }
-    return window;
-  }
-
 }