testing: Add API to close system web app

This change adds an API to InteractiveAshTest that closes the system web
app matching the provided type if it is open.

Bug: b/338645411
Change-Id: I01c314ac9f032bdbd582b47c7bfb9b081ddabd84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5605958


Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Chad Duffin <chadduffin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312123}