Automate upload of proguard mapping.txt

[tobrun]

Currently firebase crash shows obfucated stacktraces due to usage of proguard in release mode.
This is now fixed for the latest released by uploading the mapping.txt manually.

We need to look into uploading the mapping.txt automatically as part of the publish CI workflow.

[langsmith]

@tobrun , I'm not exactly sure how/where we'd set this up, but I did some research and found some links that I think are pretty helpful. They're all related to Bitrise. I couldn't really find anything related to CircleCI:

http://stackoverflow.com/a/41813757

https://github.com/Triple-T/gradle-play-publisher/issues/186#issue-206744583

https://discuss.bitrise.io/t/google-play-deploy-upload-mapping-txt/354

• https://github.com/bitrise-steplib/steps-google-play-deploy

[zugaldia]

Maybe this is something @Guardiola31337 can help us look into.