[User Education] Move per-app to per-key promo.

This is to support per-account login/logout promos.

The CL does the following:
 - Changes "kPerApp" to "kKeyedNotice" IPH subtype.
   - This remains allowlisted as it is higher-priority.
 - Moves key generation from browser-specific FeaturePromoController
   implementation to FeaturePromoParams.
 - Migrates existing per-app promo to the new system.
 - Updates tests.

Change-Id: I7b5d50cdf80121334d8fdab373eeeb23f98f1b0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404781
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Mickey Burks <mickeyburks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283262}