title: startForeground fail after upgrade to Android 8.1
body: <p>After upgrading my phone to 8.1 Developer Preview my background service no longer starts up properly.</p>
  <p>In my long-running service I've implemented a <code>startForeground</code> method to start the ongoing notification which is called in on create.</p>
  <pre><code>@TargetApi(Build.VERSION_CODES.O)
  private fun startForeground() {
      // Safe call, handled by compat lib.
      val notificationBuilder = NotificationCompat.Builder(this, DEFAULT_CHANNEL_ID)
  
      val notification = notificationBuilder.setOngoing(true)
              .setSmallIcon(R.drawable.ic_launcher_foreground)
              .build()
      startForeground(101, notification)
  }
  </code></pre>
  <p>Error message:</p>
  <pre><code>11-28 11:47:53.349 24704-24704/$PACKAGE_NAMEE/AndroidRuntime: FATAL EXCEPTION: main
      Process: $PACKAGE_NAME, PID: 24704
      android.app.RemoteServiceException: Bad notification for startForeground: java.lang.RuntimeException: invalid channel for service notification: Notification(channel=My channel pri=0 contentView=null vibrate=null sound=null defaults=0x0 flags=0x42 color=0x00000000 vis=PRIVATE)
          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1768)
          at android.os.Handler.dispatchMessage(Handler.java:106)
          at android.os.Looper.loop(Looper.java:164)
          at android.app.ActivityThread.main(ActivityThread.java:6494)
          at java.lang.reflect.Method.invoke(Native Method)
          at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
  </code></pre>
  <p><code>invalid channel for service notification</code>, apparently my old channel the <code>DEFAULT_CHANNEL_ID</code> is no longer appropriate for API 27 I assume. What would be the proper channel? I've tried to look through the documentation</p>