[Search aggregator policy] Handle user set keyword conflicts

This CL handles conflicts when the EnterpriseSearchAggregatorSettings keyword (shortcut) is equal to an existing user set keyword. We use the following rules to handle this:
- Keep the user set keyword if it does not start with @.
- Hide the user set keyword if starts with @.

The following is the behavior before and after this change:
Before (user set keyword is replaced with policy keyword): https://drive.google.com/file/d/18a3i6lU1e5JWNJZBcoSFRIYcIEKKAnp0/view?usp=sharing&resourcekey=0-j-gQZYJNPvC9D2dAY-QU4A
After (user non-featured user set keyword is kept): https://drive.google.com/file/d/175GCSJCAIVrBEKIDcCD_4S-aKcOHSNiK/view?usp=sharing&resourcekey=0-DhWk-8CSKPUgS9yaj0mWhw

Bug: 378083149
Change-Id: I5466f3675ee75296c38a39ba9dc381da0323c83d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6017072
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Commit-Queue: Ananya Seelam <ananyaseelam@google.com>
Cr-Commit-Position: refs/heads/main@{#1388910}