Also, you're recommended to set default values to customize the appearance of notifications. You can specify a custom default icon and a custom default color that are applied whenever equivalent values are not set in the notification payload.
Add these lines inside the application tag to set the custom default icon and custom color:
Set custom default icon. This is used when no icon is set for incoming notification messages.
<meta-data
android:name="com.google.firebase.messaging.default_notification_icon"
android:resource="@drawable/ic_stat_ic_notification" />
Set color used with incoming notification messages. This is used when no color is set for the incoming
notification message.
<meta-data
android:name="com.google.firebase.messaging.default_notification_color"
android:resource="@color/colorAccent" />
you may read more at this FCM Recieve Notifications