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.

从 Python 的 locale 模块以及 issue 中链接的 Android context 指南开始；没有指定具体文件或测试。确定如何访问 Android 当前的 locale，并验证 locale.getlocale() 会反映该 locale，而不是始终返回 en_US，同时保持与 locale.setlocale 的兼容性。