BluetoothRemoteGATTCharacteristic：startNotifications() 方法
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
BluetoothRemoteGATTCharacteristic.startNotifications() 方法在存在活动通知时返回一个 Promise，其会兑现为 BluetoothRemoteGATTCharacteristic 实例。