Document threading guarantee of Player.addListener

These guarantees were documented for `EventListener` and
hold as well for Listener but were not documented.

PiperOrigin-RevId: 382498188

library/common/src/main/java/com/google/android/exoplayer2/Player.java

@@ -1262,9 +1262,11 @@ public interface Player {
   Looper getApplicationLooper();
 
   /**
-   * Registers 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.
+   * Registers a listener to receive events from the player.
+   *
+   * <p>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.
    *
    * @param listener The listener to register.
    * @deprecated Use {@link #addListener(Listener)} and {@link #removeListener(Listener)} instead.
@@ -1275,6 +1277,10 @@ public interface Player {
   /**
    * Registers a listener to receive all events from the player.
    *
+   * <p>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.
+   *
    * @param listener The listener to register.
    */
   void addListener(Listener listener);