[native_toolchain_c] Consider ANDROID_HOME environment variable

[simolus3]

When searching for an NDK, this now considers:

• $ANDROID_HOME/ndk/*/ directories.
• If the environment variables ANDROID_NDK, ANROID_NDK_HOME, ANDROID_NDK_LATEST_HOME or ANDROID_NDK_ROOT are set (as they are in GitHub actions), those directories.

To make this testable (it's kind of odd there's no way to patch Platform.environment with IOOverrides), this replaces the logger passed to tool resolvers with a ToolResolvingContext class that allows mocking environment variables (well, kind of. Most resolvers shell out and we can't mock that yet).

Closes #2632.

[dcharkes]

• If the environment variables ANDROID_NDK, ANROID_NDK_HOME, ANDROID_NDK_LATEST_HOME or ANDROID_NDK_ROOT are set (as they are in GitHub actions), those directories.

Some of these are filtered out in the build hooks, so we'd need to add those there too.

Thanks @simolus3! 🚀

[github-actions]

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[simolus3]

It looks like the failing health checks may be an analyzer issue? Or at least I don't understand how these changes could cause the error from the logs.

[dcharkes]

It looks like the failing health checks may be an analyzer issue? Or at least I don't understand how these changes could cause the error from the logs.

I have filed dart-lang/sdk#61870. We're not using any dot-shorthands in the code yet, so it's kinda weird.

The new tests fail on Windows, backslashes. 🙃