[iOS] Make DisplaySwitchProfile() bool-valued

In the past, the integer value of the experimental flag
"DisplaySwitchProfile" determined how many test profiles were created.
That's not the case anymore: No more test profiles; the flag just
controls whether the developer-mode "switch profile" UI shows up in
settings.
So this CL changes the flag to only have two possible values, "enabled"
or "disabled". To avoid a migration (which isn't worth the hassle), the
underlying NSUserDefaults entry remains integer-valued.

Bug: 331783685
Change-Id: I386c6980bc358d579c32c7baf3a35ea934599983
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5934428
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1369245}