Build and release pipeline

[rtibbles]

This PR finalizes the Github Action to handle:

• Code signing of assets for development and release builds
• Proper versioning of assets, independent of action build numbers, to ensure that no matter where the workflow is invoked, it will properly increment the Android Version code - it does this by getting the max version code on the play store and incrementing it by 1
• Building and uploading an AAB to the Play Store for a release - it will then download a generated Universal APK from the Play Store to be the output APK asset.
• Adds an additional reusable workflow for rolling out an already uploaded version to the open testing track

For a complete spec of this work, see here: #151

Opening as a draft for now, as I have not had a chance to test a lot of this yet.

One last thing that probably needs to be done, is to do semver comparisons of the versionNames of the releases. This is necessary, as we do not want to release a Kolibri patch/minor release when a newer minor/major release already exists.

[rtibbles]

Have now tested this locally - had to make a few tweaks to ensure proper upload and download, but the 0.16.0b2 was successfully put on the internal testing track on the play store, and the generated universal APK subsequently downloaded.