[iOS][Omnibox][Textfield] Fix text alignment in RTL

Fix text alignment in mix RTL/LTR.

The defaultTextAttributes from UITextField contains writing direction
and alignments attributes. These attributes are updated by UIKit when
the text is updated.
In pre edit state handling, modify the attributes from the existing
UIKit dictionary instead of creating a new one.

Also:
- Returns the textfield's semanticContentAttribute when text direction
  is NSWritingDirectionNatural.
- Exit pre edit in will change to avoid overriding the text attributes
  from the new text.

Bug: 1504536
Change-Id: I085ee23b5a513dcc104e0e5b0d24cc9e9025154e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5259018
Auto-Submit: Christian Xu <christianxu@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Christian Xu <christianxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1255082}

• ios/chrome/browser/ui/omnibox/omnibox_text_field_experimental.mm[diff]
• ios/chrome/browser/ui/omnibox/omnibox_view_ios.mm[diff]