Add pseudo focused state to PickerItemView.

Add pseudo focused state to indicate that a PickerItemView responds to
certain user events, e.g. press enter to select item.

We add this pseudo focused state since the item might not have real
view focus, which generally stays on the Picker search field to allow
the user to easily type and modify their search query.

For now, add the same basic pseudo focus styling to all items, based on
the focus indicator specs for Picker list items and search field.
Followups will polish the styling and add accessibility announcements.

Bug: b:322900302
Change-Id: I9529df5bc5070244da62f15ab0abfe9abed1cbee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5321190
Commit-Queue: Michelle Chen <michellegc@google.com>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1265616}