android: Add experiment for AndroidOverlay

[borongc]

Previously, the CobaltUsingAndroidOverlay feature was unconditionally
enabled. This change moves its activation behind a configurable
Java switch.

This allows for experimental rollout and A/B testing of the
AndroidOverlay functionality, providing better control over its
deployment and evaluation on various Android devices for punch-out
video.

Issue: 476418731

[github-actions]

🤖 Gemini Suggested Commit Message

---

android: Add experiment for AndroidOverlay

Previously, the CobaltUsingAndroidOverlay feature was unconditionally
enabled. This change moves its activation behind a configurable
Java switch.

This allows for experimental rollout and A/B testing of the
AndroidOverlay functionality, providing better control over its
deployment and evaluation on various Android devices for punch-out
video.

Bug: 476418731

---

💡 Pro Tips for a Better Commit Message:

1. Influence the Result: Want to change the output? You can write custom prompts or instructions directly in the Pull Request description. The model uses that text to generate the message.
2. Re-run the Generator: Post a comment with: /generate-commit-message

[gemini-code-assist]

Code Review

This pull request introduces an experiment flag for enabling AndroidOverlay. The changes remove the hardcoded default enablement and add logic to enable it via a new EnableAndroidOverlay Java switch.

My review found a critical issue in JavaSwitches.java where a typo in a constant name would lead to a compilation failure. I've provided a suggestion to fix it and align the code with the style guide.