Use contextual snackbar text when activating a theme from preview

[dhananjaykuber]

Fixes #56876

What?

This PR updates the success notification message displayed when activating a theme via the Site Editor. Previously, the message showed "Site updated.", which has now been corrected to "Theme activated." for better clarity.

Why?

The current message, "Site updated.", can be misleading as it does not accurately convey the action taken when activating a theme. This change improves the user experience by providing a clearer and more appropriate notification.

Testing Instructions

1. Go to Appearance --> Themes
2. Click on Live Preview of any theme
3. In left bottom corner of Site Editor click on Activate {Theme_Name} button
4. Check the snackbar with updated message "Theme activated"

Before	After

[github-actions]

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Co-authored-by: dhananjaykuber <dhananjaykuber@git.wordpress.org>
Co-authored-by: carolinan <poena@git.wordpress.org>
Co-authored-by: richtabor <richtabor@git.wordpress.org>

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[carolinan]

This change updates the text in the notice everywhere, even if it is not a theme that is activated.
For example when changing the content of the Site Title in the Site Editor.

The new message needs to be limited to the theme activation.

[carolinan]

Also there is a test that would need to be updated: \test\e2e\specs\site-editor\activate-theme.spec.js