unity, reduce android size

参考：

https://www.youtube.com/watch?v=TYSmf_zgtZo

http://stackoverflow.com/questions/41087220/how-to-use-stripping-level-il2cpp-option-in-android-player-settings-unity

http://gamedev.stackexchange.com/questions/116027/armv7-vs-fat-in-unity-android-build-settings-release-multiple-valid-apks

https://docs.unity3d.com/Manual/class-PlayerSettingsAndroid.html