Add sample XML to GameService javadoc to show how to request BIND_GAME_SERVICE permission.

 * when a game session should begin. It is always kept running by the system.

 * Because of this it should be kept as lightweight as possible.

 *

 * Heavy weight operations (such as showing UI) should be implemented in the

 * <p>Heavyweight operations (such as showing UI) should be implemented in the

 * associated {@link GameSessionService} when a game session is taking place. Its

 * implementation should run in a separate process from the {@link GameService}.

 *

 * <p>To extend this class, you must declare the service in your manifest file with

 * the {@link android.Manifest.permission#BIND_GAME_SERVICE} permission

 * and include an intent filter with the {@link #SERVICE_INTERFACE} action. For example:

 * <pre>

 * <service android:name=".GameService"

 *          android:label="@string/service_name"

 *          android:permission="android.permission.BIND_GAME_SERVICE">

 *     <intent-filter>

 *         <action android:name="android.service.games.GameService" />

 *     </intent-filter>

 * </service>

 * </pre>

 *

 * @hide

 */

@SystemApi