[Android] FileUtils::listFiles implementation that works with empty folders

[rh101]

Describe your changes

This is an attempt to fix the issue of empty folder names missing from FileUtils::listFiles() on Android when the path is in the assets folder (within the APK).

This now uses the Java implementation of AssetManager.list(), which does return all file and folder names. This will then use
AssetManager.open() to check if the path is a file or folder, but if anyone knows of a better way to handle this, then please state what it is.

Files in the assets folder will be relative to that folder, and the "assets/" prefix will not be added to them, since it is not required.

Issue ticket number and link

#2458

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