AOT in Xamarin results in very large DLL in v2.11.1+

**Description**

I am using this SDK in a Xamarin.Android project.
Starting with 2.11.1, the DocumentFormat.OpenXml.dll in my release bundle grew from ~6MB to over 20MB, causing the apps download size to grow about 3MB

**Information**

- .NET Target: Xamarin
- DocumentFormat.OpenXml Version: 2.11.1 + 2.11.2

**Repro**

Xamarin.Android project with AOT+LLVM enabled
Example project: [App2.zip](https://github.com/OfficeDev/Open-XML-SDK/files/4911544/App2.zip)
You can reproduce the problem by building the Release APK with v2.11.0 and with v2.11.2 - you will notice that with v2.11.0, the uncompressed DocumentFormat.OpenXml.dll in the APK (which can be renamed to .zip and inspected) is ~15MB and with v2.11.2 it is **over 30MB**.

This libraries DLL is already by far the largest of all the libraries that I am using in this project. It would be great if it wouldn't further increase my apps size :)