[Nearby] Add pref to control Fast Init Notification

This CL adds a pref to enable/disable fast initiation notification.
This shows a notification when a nearby device is trying to share.
When the user clicks on the notification they enter high-visibility
mode and can be discovered by non-contacts.

Bug: 1223888
Change-Id: Iad9079f4da480cb4380804bdee53f9220555ebc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3114269
Commit-Queue: Claude van der Merwe <cvandermerwe@google.com>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Michael Hansen <hansenmichael@google.com>
Reviewed-by: Josh Nohle <nohle@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#917108}