There is no sound when notification is received. Android 8.

Hi, I'm trying to use sdk-26 branch. I test it on a devices and emulator. Everything is fine on android from 4.2 to 7. But on android 8 notifications appears on the screen but there is no sound.

I'm trying to send custom notification.

```
{
token: deviceToken,
data: {
custom_notification: JSON.stringify({
title: "Title",
body: "Body",
sound: "default",
priority: "high",
ticker: "Ticker",
channel: "default"
})
}
}
```

"react": "16.0.0-alpha.12",
"react-native": "0.47.1",
"react-native-fcm": "github:evollu/react-native-fcm#sdk-26"