Add missing thread verification to SimpleExoPlayer.release().

This call also needs to be made from the right thread to prevent messages
from getting lost and causing ANRs.

PiperOrigin-RevId: 239564573

library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java

@@ -980,6 +980,7 @@ public class SimpleExoPlayer extends BasePlayer
 
   @Override
   public void release() {
+    verifyApplicationThread();
     audioFocusManager.handleStop();
     player.release();
     removeSurfaceCallbacks();