Add triggers to .github/workflows/

[cartland]

This PR standardizes GitHub Actions triggers in workflow files within .github/workflows/.

The goal is to ensure workflows run consistently on push, pull_request, and workflow_dispatch events where appropriate.

This is part of a batch of pull requests across repositories owned by the android organization on GitHub.

Project Owner: Please review the changes carefully to ensure they are correct and appropriate for this project before approving and merging.

• If you do not think this change is appropriate (e.g., a workflow should NOT run on one of these triggers), please leave a comment explaining why.
• If you think the goal is appropriate but notice a mistake in the implementation, please leave a comment detailing the mistake.

[gemini-code-assist[bot]]

[!NOTE] The files in this pull request are not supported by Gemini Code Assist.

[cartland]

CI is not working on this repository, likely due to this issue:

The ubuntu-18.04 environment is deprecated, consider switching to ubuntu-20.04(ubuntu-latest), or ubuntu-22.04 instead. For more details see https://github.com/actions/virtual-environments/issues/6002

I will try to update the environment to ubuntu-20.04.

[cartland]

ubuntu-20.04 also has a deprecation plan. Switching to ubuntu-latest so that we need to do fewer updates on our end. Since we're required to update so often, CI will probably be more stable with latest than a specific version number.

[cartland]

@yrezgui I fixed the builds.