Add a test to enforce `Player` remains unstable-for-implementors

This ensures that anybody implementing `Player` (which is relatively
unusual) must override at least one `@UnstableApi` method, and therefore
opt-in to the unstable API.

PiperOrigin-RevId: 521769675

libraries/common/src/test/java/androidx/media3/common/PlayerTest.java

+/*
+ * Copyright 2023 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package androidx.media3.common;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import java.lang.reflect.Method;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+/** Tests for {@link Player}. */
+@RunWith(AndroidJUnit4.class)
+public class PlayerTest {
+
+  /**
+   * This test picks a method on the {@link Player} interface that is known will never be
+   * stabilised, and asserts that it is required to be implemented (therefore enforcing that {@link
+   * Player} is unstable-for-implementors). If this test fails because the {@link Player#next()}
+   * method is removed, it should be replaced with an equivalent unstable, unimplemented method.
+   */
+  @Test
+  public void testAtLeastOneUnstableUnimplementedMethodExists() throws Exception {
+    Method nextMethod = Player.class.getMethod("next");
+    assertThat(nextMethod.isDefault()).isFalse();
+  }
+}