Move central publishing to GH actions

[simolus3]

This simplifies the Android build and prepares the adoption of the new uploading API to maven central.

1. Building the Android library has been moved into a separate action that is called both from the regular CI and the release flow. It will collect files to be uploaded as an artifact.
2. To publish to central, we know download that artifact and upload it with curl as shown here.

For this to work, the publisher would have to be migrated and we'd have to generate a publishing token. The GH actions secret CENTRAL_AUTH should be set to printf "%s:%s" CENTRAL_USER CENTRAL_TOKEN | base64.