CI for Android app

Provide pipeline template that can be easily adapted for Android projects, focusing on both application and library development.

Ideas for a robust template:
- [Gradle](https://developer.android.com/build)
- Well configured Gradle build environment
- Exercise Gradle push and pull from a private repo
- System test
- Wrap this up into a Buildkite pipeline and get it pushing the build artifacts to the private repo [using Buildkite OIDC tokens](https://buildkite.com/docs/agent/v3/cli-oidc)
- Could we expand this for building a library for Android? Can we rig up that build and push packages from within pipelines?

Start by reviewing the repository's existing pipeline templates and the Android build guidance linked in the issue. Determine the scope for application and library builds, private repository access, system tests, artifact publishing, and Buildkite OIDC before defining what a complete adaptable template would include.

Written by the indexing model from the issue text.