Add getPlaybackLooper() to ExoPlayer v2.

A few components in ExoPlayer requires playback looper to operate (such as:
DrmSessionManager#acquireSession), so this CL add back getPlaybackLooper()
to facilitate such cases.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=159416012

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayer.java

@@ -284,6 +284,13 @@ public interface ExoPlayer {
   int REPEAT_MODE_ALL = 2;
 
   /**
+   * Gets the {@link Looper} associated with the playback thread.
+   *
+   * @return The {@link Looper} associated with the playback thread.
+   */
+  Looper getPlaybackLooper();
+
+  /**
    * Register a listener to receive events from the player. The listener's methods will be called on
    * the thread that was used to construct the player. However, if the thread used to construct the
    * player does not have a {@link Looper}, then the listener will be called on the main thread.

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java

@@ -107,6 +107,11 @@ import java.util.concurrent.CopyOnWriteArraySet;
   }
 
   @Override
+  public Looper getPlaybackLooper() {
+    return internalPlayer.getPlaybackLooper();
+  }
+
+  @Override
   public void addListener(EventListener listener) {
     listeners.add(listener);
   }

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2;
 
 import android.os.Handler;
 import android.os.HandlerThread;
+import android.os.Looper;
 import android.os.Message;
 import android.os.Process;
 import android.os.SystemClock;
@@ -270,6 +271,10 @@ import java.io.IOException;
     internalPlaybackThread.quit();
   }
 
+  public Looper getPlaybackLooper() {
+    return internalPlaybackThread.getLooper();
+  }
+
   // MediaSource.Listener implementation.
 
   @Override

library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java

@@ -481,6 +481,11 @@ public class SimpleExoPlayer implements ExoPlayer {
   // ExoPlayer implementation
 
   @Override
+  public Looper getPlaybackLooper() {
+    return player.getPlaybackLooper();
+  }
+
+  @Override
   public void addListener(EventListener listener) {
     player.addListener(listener);
   }