Stop save-mode exports landing as (invalid).json

[HugeFrog24]

Every save-mode export through the selector lands as (invalid).json, and nobody chose that name. Here is how it happens:

1. Canvas builds the save intent with a category, a MIME type, and a serene absence of any file name..
2. The picker opens with the name field blank. The user shrugs and saves anyway.
3. The provider is asked to create a document called "".
4. AOSP refuses an empty name on a FAT-safe filesystem, so it swaps in a placeholder. That placeholder is the word (invalid). It is now the user's file name.
5. Export again and the counter kicks in, so the folder fills with (invalid)(1).json, (invalid)(2).json.

The mod author gets the bug report and can do nothing with it, as the name was decided three layers below them. One extra on the intent fixes it.