Fix nullness annotation on equals methods

PiperOrigin-RevId: 286154938

library/core/src/test/java/com/google/android/exoplayer2/offline/DownloadManagerTest.java

@@ -18,6 +18,7 @@ package com.google.android.exoplayer2.offline;
 import static com.google.common.truth.Truth.assertThat;
 
 import android.net.Uri;
+import androidx.annotation.Nullable;
 import androidx.test.core.app.ApplicationProvider;
 import androidx.test.ext.junit.runners.AndroidJUnit4;
 import com.google.android.exoplayer2.C;
@@ -714,7 +715,7 @@ public class DownloadManagerTest {
     }
 
     @Override
-    public boolean equals(Object o) {
+    public boolean equals(@Nullable Object o) {
       if (this == o) {
         return true;
       }