[IOS] Enable Add to Home by default

Also disable the feature in incognito as this is the default launched option.

Change-Id: I855f94c96e87dcb2908425dfc97ea16e7fc350fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4556384


Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Auto-Submit: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148386}