ADD Github Action to upload prod build to Google Play Console

[ninovanhooff]

If there are no build flavors yet, we can just upload the default flavor.

This action still seems to be most popular: https://github.com/marketplace/actions/upload-android-release-to-play-store

See the native template implementation:

• https://github.com/Q42/Template.Android/blob/develop/.github/workflows/playstore-internal.yml
• https://github.com/Q42/Template.Android/blob/develop/README.MD