Make remaining media source reusable.

This make sure all media sources can be reprepared after being released.

Issue:#3498

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183990416

library/core/src/main/java/com/google/android/exoplayer2/source/ClippingMediaSource.java

@@ -133,7 +133,6 @@ public final class ClippingMediaSource extends CompositeMediaSource<Void> {
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
     super.prepareSource(player, isTopLevelSource, listener);
-    Assertions.checkState(sourceListener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     sourceListener = listener;
     prepareChildSource(/* id= */ null, mediaSource);
   }
@@ -162,6 +161,13 @@ public final class ClippingMediaSource extends CompositeMediaSource<Void> {
   }
 
   @Override
+  public void releaseSource() {
+    super.releaseSource();
+    clippingError = null;
+    sourceListener = null;
+  }
+
+  @Override
   protected void onChildSourceInfoRefreshed(
       Void id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest) {
     if (clippingError != null) {

library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java

@@ -88,7 +88,6 @@ public final class ConcatenatingMediaSource extends CompositeMediaSource<Integer
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
     super.prepareSource(player, isTopLevelSource, listener);
-    Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
     boolean[] duplicateFlags = buildDuplicateFlags(mediaSources);
     if (mediaSources.length == 0) {
@@ -120,6 +119,13 @@ public final class ConcatenatingMediaSource extends CompositeMediaSource<Integer
   }
 
   @Override
+  public void releaseSource() {
+    super.releaseSource();
+    listener = null;
+    timeline = null;
+  }
+
+  @Override
   protected void onChildSourceInfoRefreshed(
       Integer sourceFirstIndex,
       MediaSource mediaSource,

library/core/src/main/java/com/google/android/exoplayer2/source/LoopingMediaSource.java

@@ -63,7 +63,6 @@ public final class LoopingMediaSource extends CompositeMediaSource<Void> {
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, final Listener listener) {
     super.prepareSource(player, isTopLevelSource, listener);
-    Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
     prepareChildSource(/* id= */ null, childSource);
   }
@@ -82,6 +81,13 @@ public final class LoopingMediaSource extends CompositeMediaSource<Void> {
   }
 
   @Override
+  public void releaseSource() {
+    super.releaseSource();
+    listener = null;
+    childPeriodCount = 0;
+  }
+
+  @Override
   protected void onChildSourceInfoRefreshed(
       Void id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest) {
     childPeriodCount = timeline.getPeriodCount();

library/core/src/main/java/com/google/android/exoplayer2/source/MergingMediaSource.java

@@ -20,12 +20,12 @@ import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.upstream.Allocator;
-import com.google.android.exoplayer2.util.Assertions;
 import java.io.IOException;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 
 /**
  * Merges multiple {@link MediaSource}s.
@@ -100,7 +100,6 @@ public final class MergingMediaSource extends CompositeMediaSource<Integer> {
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
     super.prepareSource(player, isTopLevelSource, listener);
-    Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
     for (int i = 0; i < mediaSources.length; i++) {
       prepareChildSource(i, mediaSources[i]);
@@ -133,6 +132,18 @@ public final class MergingMediaSource extends CompositeMediaSource<Integer> {
   }
 
   @Override
+  public void releaseSource() {
+    super.releaseSource();
+    listener = null;
+    primaryTimeline = null;
+    primaryManifest = null;
+    periodCount = PERIOD_COUNT_UNSET;
+    mergeError = null;
+    pendingTimelineSources.clear();
+    Collections.addAll(pendingTimelineSources, mediaSources);
+  }
+
+  @Override
   protected void onChildSourceInfoRefreshed(
       Integer id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest) {
     if (mergeError == null) {