Improve robustness of ImaAdsLoader

Remove an assertion that there was a call to pause content between two
content -> ad transitions.

Also, only use the player position for resuming an ad on reattaching if the
player is currently playing an ad, in case IMA pauses content before the player
actually transitions to an ad.

Issue: #3430

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176365842

RELEASENOTES.md

@@ -53,6 +53,9 @@
     ([#3303](https://github.com/google/ExoPlayer/issues/3303)).
   * Ignore seeks if an ad is playing
     ([#3309](https://github.com/google/ExoPlayer/issues/3309)).
+  * Improve robustness of `ImaAdsLoader` in case content is not paused between
+    content to ad transitions
+    ([#3430](https://github.com/google/ExoPlayer/issues/3430)).
 * UI:
   * Allow specifying a `Drawable` for the `TimeBar` scrubber
     ([#3337](https://github.com/google/ExoPlayer/issues/3337)).

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

@@ -260,7 +260,7 @@ public void attachPlayer(ExoPlayer player, EventListener eventListener, ViewGrou
   @Override
   public void detachPlayer() {
     if (adsManager != null && imaPausedContent) {
-      adPlaybackState.setAdResumePositionUs(C.msToUs(player.getCurrentPosition()));
+      adPlaybackState.setAdResumePositionUs(playingAd ? C.msToUs(player.getCurrentPosition()) : 0);
       adsManager.pause();
     }
     lastAdProgress = getAdProgress();
@@ -628,7 +628,6 @@ private void updateImaStateForPlayerState() {
       if (!wasPlayingAd && playingAd) {
         int adGroupIndex = player.getCurrentAdGroupIndex();
         // IMA hasn't sent CONTENT_PAUSE_REQUESTED yet, so fake the content position.
-        Assertions.checkState(fakeContentProgressElapsedRealtimeMs == C.TIME_UNSET);
         fakeContentProgressElapsedRealtimeMs = SystemClock.elapsedRealtime();
         fakeContentProgressOffsetMs = C.usToMs(adPlaybackState.adGroupTimesUs[adGroupIndex]);
         if (fakeContentProgressOffsetMs == C.TIME_END_OF_SOURCE) {