fix(android): validate external files dir and mkdirs() for model download/import

[jonathanrao99]

Summary

Improves directory creation handling for model downloads and imports on Android.

Changes

• Added null checks for getExternalFilesDir(null)
• Validated mkdirs() instead of assuming success
• Added clearer error messages when directory creation fails

Why

Previously, the app continued after directory creation failed, leading to a vague "folder not found" error later in the process. This change fails early and provides clearer feedback.

Notes

This improves behavior on newer Android versions (including Android 16 QPR2) by handling storage access more defensively.

Closes #553

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[jonathanrao99]

Hi! This PR adds defensive handling for directory creation during model download/import.

• Checks for null getExternalFilesDir(null)
• Validates mkdirs() instead of assuming success
• Fails early with clearer error messages

This addresses the "folder not found" issue reported on Android 16 QPR2 by preventing silent directory creation failures and surfacing the root cause earlier.

[jonathanrao99]

Hi! I've now signed the Google CLA. Please let me know if everything looks good or if any further changes are needed. Thanks!