public Bitmap getBackground() {
            return mBackground;
        }
Get a background image to be displayed behind the notification content.
Contrary to the {@link NotificationCompat.BigPictureStyle}, this background
will work with any notification style.

@return the background image
@see NotificationCompat.WearableExtender#setBackground
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/compat/NotificationCompat.java