Upgrade IMA SDK dependency to 3.21.0

Call the new method AdsLoader.release() to allow the IMA SDK to dispose of its
WebView.

Issue: #7344
PiperOrigin-RevId: 339022162

RELEASENOTES.md

@@ -28,6 +28,9 @@
         enough space.
 *   Audio:
     *   Retry playback after some types of `AudioTrack` error.
+*   IMA extension:
+    *   Upgrade IMA SDK dependency to 3.21.0, and release the `AdsLoader`
+        ([#7344](https://github.com/google/ExoPlayer/issues/7344)).
 
 ### 2.12.1 (2020-10-23) ###
 

extensions/ima/build.gradle

@@ -25,7 +25,7 @@ android {
 }
 
 dependencies {
-    api 'com.google.ads.interactivemedia.v3:interactivemedia:3.20.1'
+    api 'com.google.ads.interactivemedia.v3:interactivemedia:3.21.0'
     implementation project(modulePrefix + 'library-core')
     implementation 'androidx.annotation:annotation:' + androidxAnnotationVersion
     implementation 'com.google.android.gms:play-services-ads-identifier:17.0.0'

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

@@ -871,6 +871,7 @@ public final class ImaAdsLoader
       if (configuration.applicationAdErrorListener != null) {
         adsLoader.removeAdErrorListener(configuration.applicationAdErrorListener);
       }
+      adsLoader.release();
     }
     imaPausedContent = false;
     imaAdState = IMA_AD_STATE_NONE;