BluetoothRemoteGATTServer：connected 属性
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
BluetoothRemoteGATTServer.connected 只读属性返回当此脚本执行环境连接到 this.device 时为 true 的布尔值。即使用户代理在物理上已连接，该属性也可能为 false。