Fix handling of postrolls preloading

The IMA SDK now preloads postrolls which is great as we no longer need
to rely on detecting buffering at the end of the stream to trigger
playing postrolls.

Add in the required logic to detect the period transition to playing
the postroll.

Issue: #7518
PiperOrigin-RevId: 317610682

RELEASENOTES.md

@@ -229,6 +229,8 @@
         on seeking to another position
         ([#7492](https://github.com/google/ExoPlayer/issues/7492)).
     *   Fix incorrect rounding of ad cue points.
+    *   Fix handling of postrolls preloading
+        ([#7518](https://github.com/google/ExoPlayer/issues/7518)).
 
 ### 2.11.5 (2020-06-05) ###
 

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

@@ -1098,11 +1098,19 @@ public final class ImaAdsLoader implements Player.EventListener, AdsLoader {
     }
     if (!sentContentComplete && !wasPlayingAd && playingAd && imaAdState == IMA_AD_STATE_NONE) {
       int adGroupIndex = player.getCurrentAdGroupIndex();
-      // IMA hasn't called playAd yet, so fake the content position.
-      fakeContentProgressElapsedRealtimeMs = SystemClock.elapsedRealtime();
-      fakeContentProgressOffsetMs = C.usToMs(adPlaybackState.adGroupTimesUs[adGroupIndex]);
-      if (fakeContentProgressOffsetMs == C.TIME_END_OF_SOURCE) {
-        fakeContentProgressOffsetMs = contentDurationMs;
+      if (adPlaybackState.adGroupTimesUs[adGroupIndex] == C.TIME_END_OF_SOURCE) {
+        adsLoader.contentComplete();
+        if (DEBUG) {
+          Log.d(TAG, "adsLoader.contentComplete from period transition");
+        }
+        sentContentComplete = true;
+      } else {
+        // IMA hasn't called playAd yet, so fake the content position.
+        fakeContentProgressElapsedRealtimeMs = SystemClock.elapsedRealtime();
+        fakeContentProgressOffsetMs = C.usToMs(adPlaybackState.adGroupTimesUs[adGroupIndex]);
+        if (fakeContentProgressOffsetMs == C.TIME_END_OF_SOURCE) {
+          fakeContentProgressOffsetMs = contentDurationMs;
+        }
       }
     }
   }
@@ -1221,7 +1229,7 @@ public final class ImaAdsLoader implements Player.EventListener, AdsLoader {
         && positionMs + THRESHOLD_END_OF_CONTENT_MS >= contentDurationMs) {
       adsLoader.contentComplete();
       if (DEBUG) {
-        Log.d(TAG, "adsLoader.contentComplete");
+        Log.d(TAG, "adsLoader.contentComplete from content position check");
       }
       sentContentComplete = true;
     }