[omnibox][ai-mode] Use kill switch approach for lens

What:
- Changes `IsAimM3Enabled()` to use the kill switch approach (see doc).
- Also, since this is the 1st of multiple such kill switch migrations,
  this makes a helper method
  `AimEligibilityService::GenericKillSwitchFeatureCheck()` they all can
  use. Which will also help when we need to later migrate them all to
  the '&&' approach after eligibility server launches.

Before:
It was using a variant of the '&&' approach.

Motive:
We want to auto-launch AIM features when we launch the eligibility
service, which is a bit unconventional, but makes the finch change
smaller

Details:
https://docs.google.com/document/d/1BXuGAybW1GXRymDBOrjzTefj503quP25-eZyhGx7kQ4/

Bug: 436225340
Change-Id: Iab67222b9cbf6f0de7a7315638cff52994ab03e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6967695
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1517737}