[iOS] Remove mainScreen uses in omnibox.

Remove uses of mainScreen and use the view.window.screen instead.
Add a property to the keyboard observer helper to see which screen is
the keyboard in.

This is a speculative fix for crash crbug.com/1335411.

Fix for change crrev.com/c/3768508

Bug: 1335411
Change-Id: Iffe2e5792c07d206787be9593ef7aee190761823
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3783037
Auto-Submit: Christian Xu <christianxu@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#1028282}