Use set-like behaviour for BaseDataSource listeners.

This prevents problems caused by unintended double-registration of the same
transfer listener.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=210078454

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

@@ -47,9 +47,11 @@ public abstract class BaseDataSource implements DataSource {
 
 
   @Override
   @Override
   public final void addTransferListener(TransferListener transferListener) {
   public final void addTransferListener(TransferListener transferListener) {
+    if (!listeners.contains(transferListener)) {
       listeners.add(transferListener);
       listeners.add(transferListener);
       listenerCount++;
       listenerCount++;
     }
     }
+  }
 
 
   /**
   /**
    * Notifies listeners that data transfer for the specified {@link DataSpec} is being initialized.
    * Notifies listeners that data transfer for the specified {@link DataSpec} is being initialized.