Add getTransferListener to BandwidthMeter.

This will allow the player to obtain the transfer listener used by the
bandwidth meter in order to pass it automatically to the relevant data
sources.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=199124880

RELEASENOTES.md

@@ -17,6 +17,8 @@
   ([#3972](https://github.com/google/ExoPlayer/issues/3972)).
 * HLS:
   * Allow injection of custom playlist trackers.
+* Add method to `BandwidthMeter` to return the `TransferListener` used to gather
+  bandwidth information.
 
 ### 2.8.1 ###
 

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

@@ -15,6 +15,8 @@
  */
 package com.google.android.exoplayer2.upstream;
 
+import android.support.annotation.Nullable;
+
 /**
  * Provides estimates of the currently available bandwidth.
  */
@@ -40,4 +42,10 @@ public interface BandwidthMeter {
 
   /** Returns the estimated bandwidth in bits/sec. */
   long getBitrateEstimate();
+
+  /**
+   * Returns the {@link TransferListener} that this instance uses to gather bandwidth information
+   * from data transfers. May be null, if no transfer listener is used.
+   */
+  @Nullable TransferListener<?> getTransferListener();
 }

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

@@ -171,6 +171,11 @@ public final class DefaultBandwidthMeter implements BandwidthMeter, TransferList
     return bitrateEstimate;
   }
 
+  @Override
+  public @Nullable TransferListener<?> getTransferListener() {
+    return this;
+  }
+
   @Override
   public synchronized void onTransferStart(Object source, DataSpec dataSpec) {
     if (streamCount == 0) {
@@ -206,14 +211,10 @@ public final class DefaultBandwidthMeter implements BandwidthMeter, TransferList
     sampleBytesTransferred = 0;
   }
 
-  private void notifyBandwidthSample(final int elapsedMs, final long bytes, final long bitrate) {
+  private void notifyBandwidthSample(int elapsedMs, long bytes, long bitrate) {
     if (eventHandler != null && eventListener != null) {
-      eventHandler.post(new Runnable()  {
-        @Override
-        public void run() {
-          eventListener.onBandwidthSample(elapsedMs, bytes, bitrate);
-        }
-      });
+      EventListener eventListener = this.eventListener;
+      eventHandler.post(() -> eventListener.onBandwidthSample(elapsedMs, bytes, bitrate));
     }
   }
 }