Media Gallery Has No Video Thumbnails

Expected behavior

The Media Gallery displays video thumbnails.

Actual behavior

The Media Gallery displays gray placeholders for all videos. Clicking on a video brings up the detail screen and displays a "Media Thumbnail could not be loaded" message. The network inspector shows a bunch of 403 unauthorized errors trying to fetch the thumbnails.

If I use the media picker from the editor, I do see many of the video thumbnails (not all of them, but I think that's due to the video size as discussed in https://github.com/wordpress-mobile/WordPress-Android/issues/15443).

Also, the previews loaded as expected on WPiOS 19.1.0.20220124.

Steps to reproduce the behavior

1. Log into an account with videos in the media library.
2. Tab the Media Button
3. Tap the Videos tab
4. Observe no thumbnails are shown
5. Tap on one of the videos
6. Note that the detail screen does not show the video's thumbnail and there is a toast saying "Media Thumbnail could not be loaded"

https://user-images.githubusercontent.com/4656348/151605610-d43ff5f3-8d78-4224-917c-17485ed6e7fe.mp4

Tested on Pixel 6 Pro and Pixel 3, Android 12, WPAndroid alpha-339 (1159)

Start by reproducing the Media Gallery flow on Android with an account containing videos, then inspect the thumbnail requests that return 403 errors. Trace the Media Gallery and video detail thumbnail-loading path; the work is done when video thumbnails appear in the Videos tab and detail screen without the loading-error toast.