Refactor: Update DevToolsPolicyDialog layout and extract to a separate file

This CL updates the visual layout of the DevToolsPolicyDialog.

The implementation has been extracted from `devtools_window.cc` into its
own new file (`devtools_policy_dialog.cc`) to improve code organization,
modularity, and testability.

Bug: 442892562

Change-Id: I341b3370ff8300aa2834c190260ebc7fc935aa5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7122761
Commit-Queue: Michał Kaczmarczyk <mickaczmarczyk@google.com>
Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Danil Somsikov <dsv@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1553402}