解决Developer warning for package xxx，Failed to post notification on channel "null"

Android O开始，系统不允许后台应用创建后台服务，要改为使用前台服务，并且应用5S内要调用该服务的startForeground()方法，否则系统将停止服务并报ANR异常：

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
    startForegroundService(serviceStartIntent)
} else {
    startService(serviceStartIntent)
}

调用startForeground()，需要传一个notification对象，在API 26以上中要使用Notification.Builder(Context, String) ，并且要指明 NotificationChannel 的Id. 如果不加则会提示：Developer warning for package xxx，Failed to post notification on channel “null”

/**
 * @see Service.onStartCommand
 */
override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        val channelId = "001"
        val channelName = "myChannel"
        val channel = NotificationChannel(channelId, channelName, NotificationManager.IMPORTANCE_NONE)
        channel.lightColor = Color.BLUE
        channel.lockscreenVisibility = Notification.VISIBILITY_PRIVATE
        val manager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
        manager.createNotificationChannel(channel)
        val notification: Notification

        notification = Notification.Builder(applicationContext, channelId)
                .setOngoing(true)
                .setSmallIcon(R.mipmap.ic_launcher)
                .setCategory(Notification.CATEGORY_SERVICE)
                .build()

        startForeground(101, notification)

    } else {
        startForeground(101, Notification())
    }

    return START_STICKY
}

/**
 * @see Service.onDestroy
 */
override fun onDestroy() {
    stopForeground(true)
    super.onDestroy()
}