Create wrapper class for LogSessionId.

The platform class is only available from API 31, so we need
a generic wrapper that can be used on all API levels. The wrapper
essentially provides an identifier for a player instance, so naming
it accordingly.

PiperOrigin-RevId: 408292802

library/core/src/main/java/com/google/android/exoplayer2/analytics/PlayerId.java

+/*
+ * Copyright 2021 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
+ *
+ *      http://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 com.google.android.exoplayer2.analytics;
+
+import static com.google.android.exoplayer2.util.Assertions.checkNotNull;
+import static com.google.android.exoplayer2.util.Assertions.checkState;
+
+import android.media.metrics.LogSessionId;
+import androidx.annotation.Nullable;
+import androidx.annotation.RequiresApi;
+import com.google.android.exoplayer2.util.Util;
+
+/** Identifier for a player instance. */
+public final class PlayerId {
+
+  /**
+   * A player identifier with unset default values that can be used as a placeholder or for testing.
+   */
+  public static final PlayerId UNSET =
+      Util.SDK_INT < 31 ? new PlayerId() : new PlayerId(LogSessionIdApi31.UNSET);
+
+  @Nullable private final LogSessionIdApi31 logSessionIdApi31;
+
+  /** Creates an instance for API &lt; 31. */
+  public PlayerId() {
+    this(/* logSessionIdApi31= */ (LogSessionIdApi31) null);
+    checkState(Util.SDK_INT < 31);
+  }
+
+  /**
+   * Creates an instance for API &ge; 31.
+   *
+   * @param logSessionId The {@link LogSessionId} used for this player.
+   */
+  @RequiresApi(31)
+  public PlayerId(LogSessionId logSessionId) {
+    this(new LogSessionIdApi31(logSessionId));
+  }
+
+  private PlayerId(@Nullable LogSessionIdApi31 logSessionIdApi31) {
+    this.logSessionIdApi31 = logSessionIdApi31;
+  }
+
+  /** Returns the {@link LogSessionId} for this player instance. */
+  @RequiresApi(31)
+  public LogSessionId getLogSessionId() {
+    return checkNotNull(logSessionIdApi31).logSessionId;
+  }
+
+  @RequiresApi(31)
+  private static final class LogSessionIdApi31 {
+
+    public static final LogSessionIdApi31 UNSET =
+        new LogSessionIdApi31(LogSessionId.LOG_SESSION_ID_NONE);
+
+    public final LogSessionId logSessionId;
+
+    public LogSessionIdApi31(LogSessionId logSessionId) {
+      this.logSessionId = logSessionId;
+    }
+  }
+}