[Media ML] Update MediaSessionManager#getActiveSessionsForUser

In order to make getActiveSessionsForUser, we need to remove
@UnsupportedAppUsage first.
Following UnsupportedAppUsage removal policy by adding
publicAlternatives to be gathered by internal tool and published
on the Android Developers site.

Bug: 160666400
Test: build
Change-Id: I13d639489091087a5c00e81071f36740002ffd9a

media/java/android/media/session/MediaSessionManager.java

@@ -32,6 +32,7 @@ import android.media.IRemoteVolumeControllerCallback;
 import android.media.MediaFrameworkInitializer;
 import android.media.MediaSession2;
 import android.media.Session2Token;
+import android.os.Build;
 import android.os.Bundle;
 import android.os.Handler;
 import android.os.RemoteException;
@@ -209,7 +210,10 @@ public final class MediaSessionManager {
      * @return A list of controllers for ongoing sessions.
      * @hide
      */
-    @UnsupportedAppUsage
+    // TODO: Remove @UnsupportedAppUsage
+    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, publicAlternatives = "Should only be"
+            + " used by system apps, since non-system apps cannot get other users' sessions."
+            + " Use {@link #getActiveSessions} instead.")
     public @NonNull List<MediaController> getActiveSessionsForUser(
             @Nullable ComponentName notificationListener, int userId) {
         ArrayList<MediaController> controllers = new ArrayList<MediaController>();