Support `locale` module on Android

Migrated from https://github.com/chaquo/chaquopy/issues/1150:

From https://github.com/beeware/toga/discussions/2562:

Hello, how can I recognize the current language with locale.getlocale()?
When i try it on android, it only returns ('en_US', 'UTF-8'), even if default language was set to an other language.

I don't think Android sets any of the standard LC_... environment variables, probably because it supports changing the locale within the lifetime of a single process. Instead, it provides an Android-specific API:

context.getResources().getConfiguration().getLocales().get(0)

This will return a string in the form en_US, which can be passed to locale.setlocale.

The difficult part is getting a reference to the context object. For details, see https://github.com/python/cpython/issues/90371#issuecomment-2991401118.

從 Python 的 locale 模組以及 issue 中連結的 Android context 指南開始；沒有指定具體檔案或測試。確定如何存取 Android 目前的 locale，並驗證 locale.getlocale() 會反映該 locale，而不是一律回傳 en_US，同時維持與 locale.setlocale 的相容性。