Use TrackSelector in DownloadHelper.

This adds the basic track selection capabilties (including tests).
The new capabilities are not exposed yet through the DownloadHelper implementations
and there will also be more helper methods (e.g. to select multiple audio lanuages at
once).

PiperOrigin-RevId: 224518477

library/core/src/main/java/com/google/android/exoplayer2/offline/TrackKey.java

@@ -15,6 +15,8 @@
  */
 package com.google.android.exoplayer2.offline;
 
+import android.support.annotation.Nullable;
+
 /**
  * Identifies a given track by the index of the containing period, the index of the containing group
  * within the period, and the index of the track within the group.
@@ -38,4 +40,23 @@ public final class TrackKey {
     this.groupIndex = groupIndex;
     this.trackIndex = trackIndex;
   }
+
+  @Override
+  public boolean equals(@Nullable Object other) {
+    if (this == other) {
+      return true;
+    }
+    if (other == null || getClass() != other.getClass()) {
+      return false;
+    }
+    TrackKey that = (TrackKey) other;
+    return periodIndex == that.periodIndex
+        && groupIndex == that.groupIndex
+        && trackIndex == that.trackIndex;
+  }
+
+  @Override
+  public int hashCode() {
+    return 31 * (31 * periodIndex + groupIndex) + trackIndex;
+  }
 }