Fix Android dirs raising opaque TypeError when the app storage folder is undetectable - #547

On Android, when the private app storage folder cannot be detected (no python4android, no pyjnius, and no recognizable /data/.../files entry on sys.path), every Android directory property (user_data_dir, user_config_dir, user_cache_dir, user_bin_dir, etc.) crashed with TypeError: expected str, bytes or os.PathLike object, not NoneType. The _android_folder() helper is documented to return str | None when detection fails, but each consumer blindly cast the result to str and passed it to path joining. After this fix, these properties raise a descriptive RuntimeError explaining that the base Android app folder cannot be determined, matching the repo's existing error style in unix.py.

The documented contract of _android_folder() allows None, so callers deserve an actionable error (or a catchable RuntimeError to fall back to another implementation) rather than an opaque TypeError from deep inside os.path.join. A new helper _require_android_folder() centralizes the None check with a clear message, and the four affected properties now use it. Implementation reason: minimal, no API signature change; the RuntimeError-with-msg pattern follows the existing unix.py precedent.

Validation:

• Regression on unchanged base: 17 assertion failure(s); with patch: 18 tests, exit 0.
• Full suite: base 1353 tests (exit 1); patch 1353 tests (exit 1).
• 105 existing failures remain; no new failing cases were detected.

AI assistance: implementation and independent review used Hermes with GLM 5.3. Test evidence was reproduced in clean checkouts. This does not represent a human review.
An additional model reviewed the supplied patch and evidence without executing tools.