Added createNotificationChannelAndroid feature

For Android, some notification properties are only defined programmatically, one of those is channel.
Channel can define the default behavior for notifications on Android 8.0+.
This feature was meant to bring the channel-only configurations importance and visibility:

FCMPlugin.createNotificationChannelAndroid({
  id: "urgent_alert", // required
  name: "Urgent Alert", // required
  description: "Very urgent message alert",
  importance: "high", // https://developer.android.com/guide/topics/ui/notifiers/notifications#importance
  visibility: "public", // https://developer.android.com/training/notify-user/build-notification#lockscreenNotification 
});

! Once a channel is created, it stays unchangeable until the user uninstalls the app. !

To have a notification to use the channel, you have to add to the push notification payload the key android_channel_id with the id given to createNotificationChannelAndroid (https://firebase.google.com/docs/cloud-messaging/http-server-ref#notification-payload-support)

Special thanks to @sajhu for requesting this feature.