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

@@ -47,8 +47,10 @@ public abstract class BaseDataSource implements DataSource {
 
 
   @Override
   @Override
   public final void addTransferListener(TransferListener transferListener) {
   public final void addTransferListener(TransferListener transferListener) {
-    listeners.add(transferListener);
-    listenerCount++;
+    if (!listeners.contains(transferListener)) {
+      listeners.add(transferListener);
+      listenerCount++;
+    }
   }
   }
 
 
   /**
   /**