Mahi: Adjust scroll bar to stay within the content section

This CL restricts the scroll view's scroll bar from overlapping with the
content section's rounded corners or extending into the bottom-right
corner's cutout region. This CL also changes the scroll bar from the
scroll view default to a thinner one with rounded corners, matching the
spec.

Recording: http://shortn/_o1TgW9BBgk

Fixed: b/328793911
Change-Id: I0c8a08777e2af79400c3a1a3a7cf468cebc89727
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5356860


Commit-Queue: Colin Kincaid <ckincaid@chromium.org>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270535}