refactor: sync & set_var

[Rudxain]

• Store some strings as consts, as explained here
• Since all calls to set_var specify ANDROID_SERIAL as key, I decided to define a function for it.

[deepsource-io]

Here's the code health analysis summary for commits e595973..63707f3. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Rust	❌ Failure	❗ 3 occurences introduced 🎯 2 occurences resolved	View Check ↗
Test coverage	⚠️ Artifact not reported	Timed out: Artifact was never reported	View Check ↗

---

💡 If you’re a repository administrator, you can configure the quality gates from the settings.