[chrome][android][selection] Add padding to list menu divider

Chrome UX has requested that we add padding to the horizontal divider
in Chrome's drop-down text selection menu (similar to other menus).
The padding appears to be equal to the padding for the list menu item
in general, therefore this dimension has been abstracted and used
within `ChromeSelectionDropdownMenuDelegate` for the divider padding.

A binder for the divider has also been added to propagate the dynamic
padding.

Bug: 1428022
Change-Id: Ib8a919412f123fb0de10574f7ada4b55721beb65
Low-Coverage-Reason: TRIVIAL_CHANGE
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5014087
Reviewed-by: Peter Conn <peconn@chromium.org>
Commit-Queue: Wayne Jackson Jr. <wbjacksonjr@chromium.org>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1222280}