Screenshots are dropped, if you untick "Include hidden threads" when uploading

STR:

1. Capture a profile, using the Gecko Platform configuration.
1. Verify that you can see a screenshots track in the profile that you just captured.
1. Click "upload" at the top right of your profile.
1. On the upload doorhanger-dialog, untick "Include Hidden Threads" (but be sure to leave "Include screenshots" checked)
1. Click the Upload button on the dialog
1. Look for screenshots in the resulting web-hosted profile.

ACTUAL RESULTS:
The screenshots track is gone; screenshots seem to have been excluded in the uploaded profile.

EXPECTED RESULTS:
Screenshots should've been included, since I had the "Include Screenshots" checkbox checked.

Sample profile created with the STR (which is lacking screenshots as a result of this bug): https://share.firefox.dev/3ciYB4q

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-358)