setSmallIcon使用
/Android5+的系统通知设置：https://github.com/OneSignal/OneSignal-Xamarin-SDK/issues/65
//默认情况下，如果通知图标是彩色的话，图标是会显示成白色块；图标应该是显示的区域要白色，其他区域则是透明的，如果需要设置背景色，可以使用setColor
//有可能一些手机厂商做了自己的处理，不按默认的方式进行
https://documentation.onesignal.com/docs/customize-notification-icons#section-1-generate-icons
Android Push Notifications: Icon not displaying in notification, white square shown instead
https://clevertap.com/blog/fixing-notification-icon-for-android-lollipop-and-above/
White square in notification bar instead of app icon on Android 5 #65
https://github.com/OneSignal/OneSignal-Xamarin-SDK/issues/65
Fixed Push Notification Icon showing White Android
https://android.jlelse.eu/fixed-push-notification-icon-showing-white-android-4da865b3dc51
Fixed Push Notification Icon showing White Android
https://android.jlelse.eu/fixed-push-notification-icon-showing-white-android-4da865b3dc51
