[CrOS Bluetooth] Move sub-header sep. creation to standalone func.

This change moves the function used to create the sub-header separator
for the Quick Settings out of DetailedViewDelegate and into
TrayPopupUtils. This function does not rely on any implementation
details and allows us to create these separators from classes outside of
the DetailedViewDelegate/TrayDetailedView classes.

Bug: 1010321
Test: autoninja -C out/Default chrome
Change-Id: Ida63762aa5eed03e092a8cc9adc10d8ce88ba5b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3176840
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Gordon Seto <gordonseto@google.com>
Commit-Queue: Chad Duffin <chadduffin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#924117}