Fix content progress updates and position faking

Occasionally the player could transition from playing content to playing an ad
after IMA called playAd. The discontinuity triggered faking the content
position, and the fake position was passed to IMA when content resumed causing
the wrong ad group to be loaded. Fix this by only faking the position if the
player transitions before playAd.

Also fix the calculation of the expected ad group index for a postroll ad, and
wait for the player to transition back from ad to content before passing a
content progress update.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185994229

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

@@ -252,10 +252,9 @@ public final class ImaAdsLoader extends Player.DefaultEventListener implements A
    */
   private boolean shouldNotifyAdPrepareError;
   /**
-   * If a content period has finished but IMA has not yet sent an ad event with {@link
-   * AdEvent.AdEventType#CONTENT_PAUSE_REQUESTED}, stores the value of {@link
-   * SystemClock#elapsedRealtime()} when the content stopped playing. This can be used to determine
-   * a fake, increasing content position. {@link C#TIME_UNSET} otherwise.
+   * If a content period has finished but IMA has not yet called {@link #playAd()}, stores the value
+   * of {@link SystemClock#elapsedRealtime()} when the content stopped playing. This can be used to
+   * determine a fake, increasing content position. {@link C#TIME_UNSET} otherwise.
    */
   private long fakeContentProgressElapsedRealtimeMs;
   /**
@@ -541,18 +540,21 @@ public final class ImaAdsLoader extends Player.DefaultEventListener implements A
       contentPositionMs = fakeContentProgressOffsetMs + elapsedSinceEndMs;
       expectedAdGroupIndex =
           adPlaybackState.getAdGroupIndexForPositionUs(C.msToUs(contentPositionMs));
-    } else if (imaAdState == IMA_AD_STATE_NONE && hasContentDuration) {
+    } else if (imaAdState == IMA_AD_STATE_NONE && !playingAd && hasContentDuration) {
       contentPositionMs = player.getCurrentPosition();
       // Update the expected ad group index for the current content position. The update is delayed
       // until MAXIMUM_PRELOAD_DURATION_MS before the ad so that an ad group load error delivered
       // just after an ad group isn't incorrectly attributed to the next ad group.
       int nextAdGroupIndex =
           adPlaybackState.getAdGroupIndexAfterPositionUs(C.msToUs(contentPositionMs));
-      if (nextAdGroupIndex != expectedAdGroupIndex
-          && nextAdGroupIndex != C.INDEX_UNSET
-          && C.usToMs(adPlaybackState.adGroupTimesUs[nextAdGroupIndex]) - contentPositionMs
-              < MAXIMUM_PRELOAD_DURATION_MS) {
-        expectedAdGroupIndex = nextAdGroupIndex;
+      if (nextAdGroupIndex != expectedAdGroupIndex && nextAdGroupIndex != C.INDEX_UNSET) {
+        long nextAdGroupTimeMs = C.usToMs(adPlaybackState.adGroupTimesUs[nextAdGroupIndex]);
+        if (nextAdGroupTimeMs == C.TIME_END_OF_SOURCE) {
+          nextAdGroupTimeMs = contentDurationMs;
+        }
+        if (nextAdGroupTimeMs - contentPositionMs < MAXIMUM_PRELOAD_DURATION_MS) {
+          expectedAdGroupIndex = nextAdGroupIndex;
+        }
       }
     } else {
       return VideoProgressUpdate.VIDEO_TIME_NOT_READY;
@@ -567,12 +569,12 @@ public final class ImaAdsLoader extends Player.DefaultEventListener implements A
   public VideoProgressUpdate getAdProgress() {
     if (player == null) {
       return lastAdProgress;
-    } else if (imaAdState == IMA_AD_STATE_NONE) {
-      return VideoProgressUpdate.VIDEO_TIME_NOT_READY;
-    } else {
+    } else if (imaAdState != IMA_AD_STATE_NONE && playingAd) {
       long adDuration = player.getDuration();
       return adDuration == C.TIME_UNSET ? VideoProgressUpdate.VIDEO_TIME_NOT_READY
           : new VideoProgressUpdate(player.getCurrentPosition(), adDuration);
+    } else {
+      return VideoProgressUpdate.VIDEO_TIME_NOT_READY;
     }
   }
 
@@ -625,6 +627,9 @@ public final class ImaAdsLoader extends Player.DefaultEventListener implements A
         Log.w(TAG, "Unexpected playAd without stopAd");
         break;
       case IMA_AD_STATE_NONE:
+        // IMA is requesting to play the ad, so stop faking the content position.
+        fakeContentProgressElapsedRealtimeMs = C.TIME_UNSET;
+        fakeContentProgressOffsetMs = C.TIME_UNSET;
         imaAdState = IMA_AD_STATE_PLAYING;
         for (int i = 0; i < adCallbacks.size(); i++) {
           adCallbacks.get(i).onPlay();
@@ -923,9 +928,9 @@ public final class ImaAdsLoader extends Player.DefaultEventListener implements A
           Log.d(TAG, "VideoAdPlayerCallback.onEnded in onTimelineChanged/onPositionDiscontinuity");
         }
       }
-      if (!wasPlayingAd && playingAd) {
+      if (!wasPlayingAd && playingAd && imaAdState == IMA_AD_STATE_NONE) {
         int adGroupIndex = player.getCurrentAdGroupIndex();
-        // IMA hasn't sent CONTENT_PAUSE_REQUESTED yet, so fake the content position.
+        // 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) {
@@ -955,9 +960,6 @@ public final class ImaAdsLoader extends Player.DefaultEventListener implements A
       pendingContentPositionMs = C.TIME_UNSET;
       sentPendingContentPositionMs = false;
     }
-    // IMA is requesting to pause content, so stop faking the content position.
-    fakeContentProgressElapsedRealtimeMs = C.TIME_UNSET;
-    fakeContentProgressOffsetMs = C.TIME_UNSET;
   }
 
   private void stopAdInternal() {