Code shrinking doesn't like Class.super.defaultMethodName

Just not doing it seems simplier and more obviously correct
than suppressing the warnings in our proguard file.

Issue: #4890

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217675527

library/core/src/main/java/com/google/android/exoplayer2/upstream/DefaultDataSource.java

@@ -23,6 +23,7 @@ import com.google.android.exoplayer2.util.Log;
 import com.google.android.exoplayer2.util.Util;
 import com.google.android.exoplayer2.util.Util;
 import java.io.IOException;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.List;
 import java.util.Map;
 import java.util.Map;
 
 
@@ -261,9 +262,7 @@ public final class DefaultDataSource implements DataSource {
 
 
   @Override
   @Override
   public Map<String, List<String>> getResponseHeaders() {
   public Map<String, List<String>> getResponseHeaders() {
-    return dataSource == null
-        ? DataSource.super.getResponseHeaders()
-        : dataSource.getResponseHeaders();
+    return dataSource == null ? Collections.emptyMap() : dataSource.getResponseHeaders();
   }
   }
 
 
   @Override
   @Override

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheDataSource.java

@@ -34,6 +34,7 @@ import java.io.InterruptedIOException;
 import java.lang.annotation.Documented;
 import java.lang.annotation.Documented;
 import java.lang.annotation.Retention;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.RetentionPolicy;
+import java.util.Collections;
 import java.util.List;
 import java.util.List;
 import java.util.Map;
 import java.util.Map;
 
 
@@ -367,7 +368,7 @@ public final class CacheDataSource implements DataSource {
     // TODO: Implement.
     // TODO: Implement.
     return isReadingFromUpstream()
     return isReadingFromUpstream()
         ? upstreamDataSource.getResponseHeaders()
         ? upstreamDataSource.getResponseHeaders()
-        : DataSource.super.getResponseHeaders();
+        : Collections.emptyMap();
   }
   }
 
 
   @Override
   @Override