Release MediaSession and ExoPlayer in MediaControllerStubTest

Due to poor isolation between the session tests, in particular the
static state in `MediaSession.SESSION_ID_TO_SESSION_MAP`, an unreleased
session at the end of one test can cause subsequent tests to fail with
obscure errors like `Session ID must be unique`.

#minor-release

PiperOrigin-RevId: 600737697

libraries/session/src/test/java/androidx/media3/session/MediaControllerStubTest.java

@@ -139,5 +139,8 @@ public class MediaControllerStubTest {
         /* query= */ "",
         /* itemCount= */ 1,
         /* libraryParams= */ getThrowingBundle());
+
+    session.release();
+    player.release();
   }
 }