Add guarantied invalid command to allow niche optimisations

This allows users to use `int` to store an optional command.

PiperOrigin-RevId: 374600127

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

@@ -1097,6 +1097,7 @@ public interface Player {
   @Documented
   @Retention(RetentionPolicy.SOURCE)
   @IntDef({
+    COMMAND_INVALID,
     COMMAND_PLAY_PAUSE,
     COMMAND_PREPARE_STOP,
     COMMAND_SEEK_TO_DEFAULT_POSITION,
@@ -1169,6 +1170,9 @@ public interface Player {
   /** Command to get the text that should currently be displayed by the player. */
   int COMMAND_GET_TEXT = 23;
 
+  /** Represents an invalid {@link Command}. */
+  int COMMAND_INVALID = -1;
+
   /**
    * Returns the {@link Looper} associated with the application thread that's used to access the
    * player and on which player events are received.