Wrong analytics image source for Stock Photo in Gutenberg Editor

[ngoctr]

This PR fixes an issue where the analytics source of stock photo images being sent as Giphy within Gutenberg Editor. The issue's found when I were working on #13803.

To test

1. Go to Site -> Posts, open a Post to edit with Block Editor (you may need to switch if your default editor is the classic one)
2. Add an image block, tap ADD IMAGE then choose Free Photo Library
3. Search and select an image, then tap Add button at the bottom right.
4. Notice the message 🔵 Tracked: editor_photo_added <blog_id: 174512392, bytes: 815818752, ext: jpeg, media_origin: not_identified, megapixels: 1, mime: image/jpeg, via: stock_photos> where the via is correctly pointed to stock_photos source.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[ScoutHarris]

Nice! Thanks for fixing!