cros_next: Add a stroke at the bottom of the header view

This CL adds a stroke at the bottom of the header view to make the
adjacent border of the header and the preview more noticeable especially
when the color for the window frame header is similar to the color of
the header.

In order to layout the stroke correctly and control its visibility, add
the stroke as a child of the header view. Since we need to layout the
header view manually now, take the header view out of the window mini
view and implement a new header class.

Bug: b/283294485
Change-Id: I953ed1eb6e75c57aace827870206ccb51ab6945d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4547286


Commit-Queue: Connie Xu <conniekxu@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147595}