title: Flutter Firebase: Heads up notification not showing on background
body: <p>When receiving an FCM from cloud functions in a Flutter app, on iOS the notification appears with a banner and in the system tray as expected. However, on Android the notification appears straight in the system tray without displaying a banner, when the app is terminated or in background.</p>
  <p>I am using latest versions of flutter and firebase messaging plugin, and I set the
  <code>default_notification_channel_id</code> in the <code>AndroidManifest.xml</code>, then I used the Local Notifications Plugin to create an Android Notification Channel with the same name that I did set in <code>AndroidManifest.xml</code> and I did set <code>importance: Importance.max</code> for the channel.</p>
  <p>I spent a few days trying show Heads up notifications and I red all of the documentations and related issues, but unfortunately it is still not showing.
  Although I am using  the Local Notifications Plugin to show Heads up notifications on foreground with no problems.</p>
  <p>At last I used the Local Notifications Plugin to show the notification on <code>FirebaseMessaging.onBackgroundMessage</code> so I get the Heads up notification, but the original notification that sent by FCM is still in the notification tray.</p>
  <p>I appreciate any help.</p>
  <p><strong>Edit:</strong>
  The problem was the Android version that I am using for testing,
  Notification channels is a concept that is specific Android 8 or newer, which is why the API docs state that the method to create channels is only for those versions of Android</p>
  <p>Is there any way to show Heads up notification on background on Android 6?
  How can I the default FirebaseMessaging channel importance?</p>