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

@param background the background bitmap
@return this object for method chaining
@see NotificationCompat.WearableExtender#getBackground
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/compat/NotificationCompat.java