[Bug]: `app dev` creates the host theme for theme app extensions every time

[karreiro]

In which of these areas are you experiencing a problem?

Theme

Expected behavior

When users run app dev in a app with theme app extensions, the host theme should be created for the first time, and then re-used.

Actual behavior

Every time users run app dev in a app with theme app extensions, a new host theme is created.

Reproduction steps

1. Run app dev and wait for the host theme creation
2. Stop the process
3. Re-run app dev and notice the new theme is created