[iOS][OmniboxPosition] Add secondary action in FRE

Add a secondary action in the omnibox position choice screen in FRE.
When tapping this button, the default omnibox position is set to the
option presented by default in the screen.

When using the secondary action, the user can exit the FRE without
selecting a preferred omnibox position. To prevent the app-launch promo
from showing after the FRE, a new condition has been added to the
feature engagement configuration.

This CL:
- Sets the default value of kBottomOmnibox to change the omnibox
position.
- Sets the user value of kBottomOmniboxByDefault to save the default
position.
- Uses kBottomOmniboxByDefault in ToolbarMediator for the default
omnibox position.
- Notifies kOmniboxPositionPromoShown when the omnibox position choice
screen is shown.

Bug: 1503638
Change-Id: I3718d2764bae52cdd216ebf8e48c73dd88666bdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5116599
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Auto-Submit: Christian Xu <christianxu@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#1237654}