Fix adUiElements variables even more robust

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

@@ -68,6 +68,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
 import java.util.List;
 import java.util.Map;
 import java.util.Map;
 import java.util.Set;
 import java.util.Set;
@@ -146,7 +147,7 @@ public final class ImaAdsLoader
      * @see AdsRenderingSettings#setUiElements(Set)
      * @see AdsRenderingSettings#setUiElements(Set)
      */
      */
     public Builder setAdUiElements(Set<UiElement> adUiElements) {
     public Builder setAdUiElements(Set<UiElement> adUiElements) {
-      this.adUiElements = Assertions.checkNotNull(adUiElements);
+      this.adUiElements = new HashSet<>(Assertions.checkNotNull(adUiElements));
       return this;
       return this;
     }
     }