Default Gradients are lost on theme changes for self hosted sites.

[chipsnyder]

Describe the bug
On self-hosted sites, when you select a theme with a custom gradient then revert to a theme without customs gradients then the default gradients aren't rendered.

To Reproduce
Steps to reproduce the behavior:

1. Select a theme on your self-hosted site with custom gradients (such as twentytwenty-copy.zip)
2. In the Android app open the block editor
3. EXPECT: to see the Custom Gradients
4. Change your site's theme to a theme without custom gradients (such as Twenty-Twenty)
5. Open the block editor again
6. Error: gradients aren't populated

Expected behavior
Expect to see the list of default gradients

Screenshots

Smartphone (please complete the following information):

• Device: Pixel 3a
• OS: Android 10
• Version 29

Additional context
Discovered as part of wordpress-mobile/WordPress-Android#12258 (comment)

• In my testing, this seems to only be effecting self-hosted sites, and only on Android.