Remove AdsLoader listeners on releasing ImaAdsLoader

Issue: #4114
PiperOrigin-RevId: 227516509

RELEASENOTES.md

 # Release notes #
 
+### 2.9.4 ###
+
+* IMA extension: Clear ads loader listeners on release
+  ([#4114](https://github.com/google/ExoPlayer/issues/4114)).
+
 ### 2.9.3 ###
 
 * Captions: Support PNG subtitles in SMPTE-TT

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

@@ -597,6 +597,8 @@ public final class ImaAdsLoader
       adsManager.destroy();
       adsManager = null;
     }
+    adsLoader.removeAdsLoadedListener(/* adsLoadedListener= */ this);
+    adsLoader.removeAdErrorListener(/* adErrorListener= */ this);
     imaPausedContent = false;
     imaAdState = IMA_AD_STATE_NONE;
     pendingAdLoadError = null;