[iOS] Add App Bundle Promo Flag and FieldTrial Config

This change adds #ios-app-bundle-promo-magic-stack to chrome://flags to
enable the App Bundle Promo ephemeral card feature in the Magic Stack.
This change also adds a FieldTrial config for this feature. The feature
is explicitly disabled in some UI tests to mitigate test failures.

Fixed: 444226938
Change-Id: I0edb64c0b7ff56e1f7724e2dca72f9c13effd16d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6936303
Reviewed-by: Ginny Huang <ginnyhuang@chromium.org>
Reviewed-by: Benjamin Williams <bwwilliams@google.com>
Commit-Queue: Eric Ekey <ericekey@google.com>
Cr-Commit-Position: refs/heads/main@{#1514446}