Release with GitHub actions

Even with a shell script, releases to both PlayStore and GitHub releases are still stressful work.
There are 2 aspects:
1. Run shell scripts in a particular sequence;
2. Ensure `keys.secret` are in the right place;

It leads to long release cycle.

The ideal way is we just create a new tag that starts with `v`, then the CI server handles all of the left.

https://www.geeksforgeeks.org/automated-release-for-android-using-github-actions/
https://github.com/marketplace/actions/build-android-release-apk-when-create-new-tag
https://github.com/marketplace/actions/upload-android-release-to-play-store