[iOS] Check GeminiSettings Enterprise policy for BWG

Modifies the GeminiSettings policy to support iOS as of M139 and adds
the policy check to the BWG service's eligibility function. Adds a unit
test to check that the eligibility pref is being evaluated.

Fixed: 420909459
Change-Id: I334d2d41ed5b887ea7369044cc97ee21c04ae447
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6654930
Commit-Queue: Adam Arcaro <adamta@google.com>
Reviewed-by: Yann Dago <ydago@chromium.org>
Reviewed-by: Ernesto Izquierdo Clua <eic@google.com>
Cr-Commit-Position: refs/heads/main@{#1476577}