Show stock photos only for wpcom or jetpack sites

[marecar3]

Fixes # wordpress-mobile/gutenberg-mobile#1539 (comment)

We should not show the Free Media Library option on Self Hosted site, since this is a WPCom/Jetpack-only feature.

To test:

Test 1:

1. The easiest way is to create a Jurassic Ninja instance:
   https://jurassic.ninja
2. Then delete the Jetpack plugin from it
3. Log in with a self-hosted site
4. Create a new Post
5. Create a new Image block
6. Tap on the Image block
7. Choose from Free Photo Library shouldn't be visible

Test 2:

1. Use wp.com site
2. Create a new Post
3. Create a new Image block
4. Tap on the Image block
5. Choose from Free Photo Library should be visible

PR submission checklist:

• I have considered adding unit tests where possible.

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[hypest]

LGTM!