[send-tab-to-self/iOS] Add "no target device" state

Behavior changes are behind a feature flag disabled by default.
Mock: https://docs.google.com/presentation/d/1w4me9wINwMW2UD-uuSQGMKiWuqkX9GDfAFvSM74Pt00/edit#slide=id.g12f7f56f6ff_2_0

Bug: 1264471
Change-Id: Idad13997294bc65586342d40a8b4c73f7f0f644d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3706611


Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1025626}