[CI] Use ubuntu-18.04 instead of macOS for Android releases

Summary:
This diff modifies the Android release workflow to use Ubuntu runners instead of macOS runners.

This should speed things up as the Android release workflow is ~2x faster on Ubuntu than on macOS.

Another benefit is that GitHub Actions gives us 20 concurrent linux runners vs 5 concurrent macOS runners, which should speed things up once we're running workflows on each commit.

Test Plan: Build 116 was created using this config (on a fork of main repo) and successfully uploaded to the Google Play Console. I set it as the Open Testing build and will test the build on a physical Android device to make sure everything is okay (don't think we've ever built the android app on linux before?)

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga, benschac, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2347