[iOS] Re-fetch Gemini eligibility on account change

The BWG Service now observes the identity manager to get a callback
when the primary account changes, which is used to re-fetch Gemini
Enterprise eligibility.

Change-Id: I674cfb0186ccde8ffbfb96574fc9f0dfcfefc19e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6811943
Commit-Queue: Adam Arcaro <adamta@google.com>
Reviewed-by: Nicolas MacBeth <nicolasmacbeth@google.com>
Commit-Queue: Nicolas MacBeth <nicolasmacbeth@google.com>
Auto-Submit: Adam Arcaro <adamta@google.com>
Cr-Commit-Position: refs/heads/main@{#1495692}