Add missing TrackKey class

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=165296184

library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/manifest/TrackKey.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.source.smoothstreaming.manifest;
+
+import android.os.Parcel;
+import android.os.Parcelable;
+import android.support.annotation.NonNull;
+
+/**
+ * Uniquely identifies a track in a {@link SsManifest}.
+ */
+public final class TrackKey implements Parcelable, Comparable<TrackKey> {
+
+  public final int streamElementIndex;
+  public final int trackIndex;
+
+  public TrackKey(int streamElementIndex, int trackIndex) {
+    this.streamElementIndex = streamElementIndex;
+    this.trackIndex = trackIndex;
+  }
+
+  @Override
+  public String toString() {
+    return streamElementIndex + "." + trackIndex;
+  }
+
+  // Parcelable implementation.
+
+  @Override
+  public int describeContents() {
+    return 0;
+  }
+
+  @Override
+  public void writeToParcel(Parcel dest, int flags) {
+    dest.writeInt(streamElementIndex);
+    dest.writeInt(trackIndex);
+  }
+
+  public static final Creator<TrackKey> CREATOR = new Creator<TrackKey>() {
+    @Override
+    public TrackKey createFromParcel(Parcel in) {
+      return new TrackKey(in.readInt(), in.readInt());
+    }
+
+    @Override
+    public TrackKey[] newArray(int size) {
+      return new TrackKey[size];
+    }
+  };
+
+  // Comparable implementation.
+
+  @Override
+  public int compareTo(@NonNull TrackKey o) {
+    int result = streamElementIndex - o.streamElementIndex;
+    if (result == 0) {
+      result = trackIndex - o.trackIndex;
+    }
+    return result;
+  }
+
+}