Replace the unpinned third-party upload-google-play action with fastlane
supply from a small nix release devshell, and pin the remaining
third-party GitHub release action to a commit SHA.
Drop setup-java, setup-android and the Gradle wrapper in favour of the
flake's pinned JDK, Gradle and Android SDK, restoring the Nix store with
cache-nix-action, so the release and next builds share the dev toolchain.
Set applicationId to com.subtracks and add a GitHub Actions workflow
that, on a v* tag, builds and signs the release APK and AAB, creates
the GitHub release with the APK, and uploads the AAB to Google Play.
Release notes come from the matching metadata changelog.
Signing reads the keystore from the environment
(RELEASE_KEYSTORE, RELEASE_KEYSTORE_PASSWORD, RELEASE_KEY_ALIAS,
RELEASE_KEY_PASSWORD) and falls back to the debug key locally.
austinried