title: Flutter app on android does not open app when notification is clicked
body: <p>Following instructions on <a href="https://pub.dev/packages/firebase_messaging" rel="nofollow noreferrer">https://pub.dev/packages/firebase_messaging</a>, I have implemented FCM. Although I have set  <code>click_action: FLUTTER_NOTIFICATION_CLICK</code>, when I click on the notification tray item, it does not open the app. The data received by the flutter app from the cloud function is as follows. How do I at least open the app upon notification click?</p>
  
  <pre><code>onMessage: {notification: {title: pi, body: text message.}, 
            data: {USER_NAME: pi, click_action: FLUTTER_NOTIFICATION_CLICK, }}
  </code></pre>
  
  <p>Cloud function payload </p>
  
  <pre><code> const payload = {
          notification: {
              title: sender,
              body: content,
              clickAction: 'ChatActivity',
          },
          data: {
              "click_action": 'FLUTTER_NOTIFICATION_CLICK',
              "USER_NAME": sender,
          }
      };
  </code></pre>
  
  <p>I have also created the notification channels and AndroidManifest has been correctly configured.</p>
  
  <pre><code>private void createNotificationChannel() {
          // Create the NotificationChannel, but only on API 26+ because
          // the NotificationChannel class is new and not in the support library
          if (Build.VERSION.SDK_INT &gt;= Build.VERSION_CODES.O) {
              CharSequence name = getString(R.string.app_name);
              String description = getString(R.string.notification_channel_description);
              int importance = NotificationManager.IMPORTANCE_HIGH;
              NotificationChannel channel = new NotificationChannel(getString(R.string.default_notification_channel_id), name, importance);
              channel.setDescription(description);
              // Register the channel with the system; you can't change the importance
              // or other notification behaviors after this
              NotificationManager notificationManager = getSystemService(NotificationManager.class);
              notificationManager.createNotificationChannel(channel);
          }
      }
  </code></pre>