Move proto proguard into a library

[yschimke]

          We should move this into a library and stop sprinkling it everywhere.

Originally posted by @yschimke in #1812 (comment)

[oas004]

Is this something I can pickup maybe? Or is it very difficult and critical? :)

[yschimke]

Yeah I think so. It might be different in a library published as an AAR file.

This is an example from OkHttp for a jar library. https://github.com/square/okhttp/blob/master/okhttp/src/jvmMain/resources/META-INF/proguard/okhttp3.pro

[oas004]

I tired to look into this for a while, but I'm not really sure how to solve this. Sorry for not updating this issue. If someone else wants to pick this up, please feel free to :)