feat: add automatic Android API Level detection via getprop

[MartheusX]

Description

Currently, maturin requires the ANDROID_API_LEVEL environment variable to be explicitly set when building on-device in environments like Termux. If missing, the build fails even though the host system is a live Android instance where this information is readily available.

This PR introduces a fallback mechanism:

1. It checks if the current platform is Android.
2. If ANDROID_API_LEVEL and other clues (linker/CC) are missing, it attempts to query getprop ro.build.version.sdk.
3. This significantly improves the developer experience for users building Rust-based Python packages directly on Android devices.

Motivation

I encountered this issue while trying to install tools in Termux on my Redmi Note 8 Pro. Automating this step removes a manual barrier for many mobile developers.

Testing

• Verified on Android 11 (API 30) within Termux.
• Confirmed that manual environment variables still take priority over this fallback.

[messense]

I don't believe this is strictly required, ANDROID_API_LEVEL is not required since 1.13.0 release as long as you have clang installed which is the case when you try to develop on Termux I think.

Though it does not hurt to include this, but please verify the claim.

[MartheusX]

​"I have verified the claim on my another device (Xiaomi Poco F3, Android 13, API 33).
​As shown in the attached screenshot, clang -

dumpmachine returns aarch64-unknown-linux-android24, but the actual system API level retrieved via getprop is 33.
​Relying solely on clang clues is inaccurate for on-device builds in Termux, as clang defaults to a much lower API level (24) than the host system actually supports. My proposed getprop fallback ensures that maturin correctly identifies the host's actual capabilities (API 33) when building natively.
​Without this fallback or a manual ANDROID_API_LEVEL export, the developer is forced to manually investigate and set the correct version, which goes against the goal of making maturin 'just work'."

[messense]

So you are saying the lower API level 24 does not work on Termux, any errors? Could you also check the output of python3 -m pip debug --verbose on Termux, it should show the supported platform tags. Thanks!