[iOS] Change account switcher menu view to bottom sheet.

The final design decision of the account switcher menu view is a
pop-over on iPads and a bottom sheet on iPhones instead of an anchored
menu.
This CL updates the initial implementation to match the final design
update.

Screenshots in https://crbug.com/336719423#comment10

Bug: 336719423
Low-Coverage-Reason: TESTS_IN_SEPARATE_CL
Change-Id: Ib8713dca7c3425abe5deaa69f6c381babbe94a40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5574968
Commit-Queue: Jood Hajeer <jood@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1307367}