Make the scroll bar always visible on the QS detailed view

I added a setter to the rounded scroll bar. If this is set to
true, the scroll thumb will now always be displayed if the content
length is greater than the container in the scroll view.

See screenshots under the attached bug.

Bug: b/362534517
Change-Id: If2ce28113b200e23d7d3decba58805c3a5a5fb6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5827729


Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Jiaming Cheng <jiamingc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349497}