How to obfuscate DLL in xamarin?

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Android

Xamarin

Hi I am using Xamarin with VS2015 Community Edition.

My problem is when I build the project I can obfuscate however when I publish the apk or export the apk it will replace the obfuscated dll with original dll.
How can I obfuscate app even when I publish the apk?

What I have tried:

So for that I used unsigned apk created by exporting package in visual studio.

aapt r -v cn.apk assemblies/App.dll //remove the original dll
aapt a -v cn.apk assemblies/AppObfuscated.dll //add the obfuscated dll

Then signed the app using same key-store using jarsigner and zipaligned it. When I tried to install it will install however it wont work.