Clarify StreamKey documentation.

StreamKey makes only sense in the context of a specific manifest type and its only
used for manifest filtering.

PiperOrigin-RevId: 225360357

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

@@ -77,7 +77,7 @@ public final class DownloadAction {
    *
    * @param type The type of the action.
    * @param uri The URI of the media to be downloaded.
-   * @param keys Keys of tracks to be downloaded. If empty, all tracks will be downloaded.
+   * @param keys Keys of streams to be downloaded. If empty, all streams will be downloaded.
    * @param customCacheKey A custom key for cache indexing, or null.
    * @param data Optional custom data for this action. If {@code null} an empty array will be used.
    */
@@ -117,8 +117,8 @@ public final class DownloadAction {
   /** Whether this is a remove action. If false, this is a download action. */
   public final boolean isRemoveAction;
   /**
-   * Keys of tracks to be downloaded. If empty, all tracks will be downloaded. Empty if this action
-   * is a remove action.
+   * Keys of streams to be downloaded. If empty, all streams will be downloaded. Empty if this
+   * action is a remove action.
    */
   public final List<StreamKey> keys;
   /** A custom key for cache indexing, or null. */
@@ -130,8 +130,8 @@ public final class DownloadAction {
    * @param type The type of the action.
    * @param uri The uri being downloaded or removed.
    * @param isRemoveAction Whether this is a remove action. If false, this is a download action.
-   * @param keys Keys of tracks to be downloaded. If empty, all tracks will be downloaded. Empty if
-   *     this action is a remove action.
+   * @param keys Keys of streams to be downloaded. If empty, all streams will be downloaded. Empty
+   *     if this action is a remove action.
    * @param customCacheKey A custom key for cache indexing, or null.
    * @param data Custom data for this action. Null if this action is a remove action.
    */
@@ -177,7 +177,7 @@ public final class DownloadAction {
     return id.equals(other.id);
   }
 
-  /** Returns keys of tracks to be downloaded. */
+  /** Returns keys of streams to be downloaded. */
   public List<StreamKey> getKeys() {
     return keys;
   }

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

@@ -19,8 +19,11 @@ import android.support.annotation.NonNull;
 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.
+ * Identifies a stream in a {@link FilterableManifest} 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.
+ *
+ * <p>Note that the interpretation of period, group and index depends on the type of manifest being
+ * filtered.
  */
 public final class StreamKey implements Comparable<StreamKey> {