Fix ImaAdsLoader @Override for unreleased method

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=202625377

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

@@ -667,7 +667,7 @@ public final class ImaAdsLoader
     }
   }
 
-  @Override
+  // TODO: Add @Override when the next IMA release is available.
   public int getVolume() {
     if (player == null) {
       return lastVolumePercentage;