Tweak discontinuity reason definitions for remote players

Discontinuity reasons may not be precisely obtained for remote
player. 'Remote Player' means that playback is owned by another
app or device and the same playback can be controller by other
clients simultaneously. The MediaController is an example.
If the remote playback doesn't provide discontinuity reason, then
player cannot differentiate between automatic playback transition
and seekTo() from another client.

This CL tweaks the discontinuity reason definitions, so reasons
can be obtained without remote playback's support.
This doesn't effect the local Players, such as SimpleExoPlayer.

PiperOrigin-RevId: 368579577

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

@@ -946,14 +946,15 @@ public interface Player {
   })
   })
   @interface DiscontinuityReason {}
   @interface DiscontinuityReason {}
   /**
   /**
-   * Automatic playback transition from one period in the timeline to the next. The period index may
-   * be the same as it was before the discontinuity in case the current period is repeated.
+   * Automatic playback transition from one period in the timeline to the next without explicit
+   * interaction by this player. The period index may be the same as it was before the discontinuity
+   * in case the current period is repeated.
    *
    *
    * <p>This reason also indicates an automatic transition from the content period to an inserted ad
    * <p>This reason also indicates an automatic transition from the content period to an inserted ad
    * period or vice versa.
    * period or vice versa.
    */
    */
   int DISCONTINUITY_REASON_AUTO_TRANSITION = 0;
   int DISCONTINUITY_REASON_AUTO_TRANSITION = 0;
-  /** Seek within the current period or to another period. */
+  /** Seek within the current period or to another period by this player. */
   int DISCONTINUITY_REASON_SEEK = 1;
   int DISCONTINUITY_REASON_SEEK = 1;
   /**
   /**
    * Seek adjustment due to being unable to seek to the requested position or because the seek was
    * Seek adjustment due to being unable to seek to the requested position or because the seek was
@@ -964,7 +965,7 @@ public interface Player {
   int DISCONTINUITY_REASON_SKIP = 3;
   int DISCONTINUITY_REASON_SKIP = 3;
   /** Discontinuity caused by the removal of the current period from the {@link Timeline}. */
   /** Discontinuity caused by the removal of the current period from the {@link Timeline}. */
   int DISCONTINUITY_REASON_REMOVE = 4;
   int DISCONTINUITY_REASON_REMOVE = 4;
-  /** Discontinuity introduced internally by the source. */
+  /** Discontinuity introduced internally (e.g. by the source). */
   int DISCONTINUITY_REASON_INTERNAL = 5;
   int DISCONTINUITY_REASON_INTERNAL = 5;
 
 
   /**
   /**