Mark PDBManager as a SystemApi

package android.service.persistentdata;

import android.annotation.SystemApi;

import android.os.RemoteException;

import android.util.Slog;

 * {@link PersistentDataBlockManager#getDataBlockSize()}.

 *

 * Clients can query the maximum size for a block via

 * {@link PersistentDataBlockManager#getMaximumDataBlockSize()}

 *

 * Clients can read the currently written block by invoking

 * {@link PersistentDataBlockManager#read()}.

 *

 * @hide

 */

@SystemApi

public class PersistentDataBlockManager {

    private static final String TAG = PersistentDataBlockManager.class.getSimpleName();

    private IPersistentDataBlockService sService;