ci: set up CI with Android builds

[boringcactus]

Summary

Ticket: Port relevant code from experiment into mobile_app repo › CI

Ports in the Android CI process from the previous repo.

Testing

Checked the Fastlane process locally.

[arkadyan]

👍

question: Several of the files in the fastlane directory are empty. Are there reasons for including those?

[arkadyan]

question: Do you think that devs will ever have to run this themselves (in which case it might be worth linking to this from the main Readme), or will it likely just be for CI?

[boringcactus]

I think the only context in which someone's likely to run this locally is if it has stopped working and they're debugging it. Fastlane generates this README automatically when run locally, and I figured I'd leave it in.

[boringcactus]

Fastlane uses those metadata files as the source of truth for the Google Play listing, so if we include the empty files now we'll know what names they should have when we start filling in the store page.