refactor(sync): remove set_var in favor of adb_id

[Rudxain]

• Since all calls to set_var specify ANDROID_SERIAL as key, I decided to define a function for it
• Now there's no implicit ANDROID_SERIAL, most fns now have to explicitly specify the device serial ID. See why env-vars are unsafe and set_var/remove_var are unsound in combination with C code that reads the environment rust-lang/rust#27970
• Define supports_multi_user helper, because of this
• Improve list_all_system_packages API
• Store some strings as consts, as explained here
• Speed optimizations, possibly at the cost of memory
• Improve documentation
• Better logging and error messages

[deepsource-io]

Here's the code health analysis summary for commits b8f6688..04b3ef5. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Rust	❌ Failure	❗ 10 occurences introduced 🎯 15 occurences resolved	View Check ↗
Test coverage	⚠️ Artifact not reported	Timed out: Artifact was never reported	View Check ↗

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[Rudxain]

Superseded by #757