title: How to add android notification channel id for flutter app to fix notification while app is on background
body: <p>In my flutter app the function onResume and onLunch not working on android platform, while they work fine on IOS,
  i receive the following message on console instead of printed strings in those functions:</p>
  
  <p>"W/FirebaseMessaging(24847): Missing Default Notification Channel metadata in AndroidManifest. Default value will be used."</p>
  
  <p>onMessage function works fine, the problem is when the app is in background</p>
  
  <p>my guess is that it has something to do with android notification channel id which should be added in android manifest</p>
  
  <p>when i add that to manifest with adding the following code to AndroidManifest the message change to :
  (I added a strings.xml file in values and defined "default_notification_channel_id" there.)</p>
  
  <p>"Notification Channel set in AndroidManifest.xml has not been created by the app. Default value will be used."</p>
  
  <pre><code>&lt;meta-data android:name="com.google.firebase.messaging.default_notification_channel_id" android:value="@string/default_notification_channel_id"/&gt;
  </code></pre>
  
  <p>in my console i should receive onResume and onLunch strings which i printed , but instead i receive following messages :</p>
  
  <p>"W/FirebaseMessaging(24847): Missing Default Notification Channel metadata in AndroidManifest. Default value will be used."</p>
  
  <p>"Notification Channel set in AndroidManifest.xml has not been created by the app. Default value will be used."</p>
  
  <pre><code>&lt;meta-data android:name="com.google.firebase.messaging.default_notification_channel_id" android:value="@string/default_notification_channel_id"/&gt;
  </code></pre>